.github/workflows/ci.yml

@@ -314,8 +314,23 @@ jobs:
     name: E2E (API 37 preview)
     runs-on: ubuntu-latest
     timeout-minutes: 35
+    # Sharded N=2 (docs/perf/api37-e2e-sharding-spike.md). The instrumented suite is split across
+    # 2 parallel API 37 emulators — each a separate matrix leg that hand-provisions its OWN emulator
+    # and runs one shard via AndroidJUnitRunner's numShards/shardIndex. This leg is the pipeline's
+    # critical path, so sharding cuts it ~17.1 min -> ~12.7 min (~28% off total CI) for +1 emulator
+    # job. FAN-IN: `ci-passed` lists `e2e-preview` once and GHA rolls BOTH shard legs under that one
+    # entry — a matrix job's aggregate result is `failure` if ANY leg fails — so both shards must
+    # pass, and branch protection (which requires the "CI passed" context, not the per-leg
+    # "E2E (API 37 preview) (N)" names) needs no change. `fail-fast: false` lets one shard's failure
+    # NOT cancel the other, so both reports always upload. Keep API37_NUM_SHARDS in lockstep with the
+    # length of matrix.shard.
+    strategy:
+      fail-fast: false
+      matrix:
+        shard: [0, 1] # 0-based shardIndex values; length must equal API37_NUM_SHARDS below
     env:
       API37_IMAGE: "system-images;android-37.0;google_apis_ps16k;x86_64"
+      API37_NUM_SHARDS: "2"
     steps:
       - name: Check out source
         uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
@@ -427,12 +442,31 @@ jobs:
             return 1
           }
 
+          # Start the adb daemon up-front (mirrors api37_e2e.py wait_for_boot) so attempt 1 can't
+          # lose the adb-server "Address already in use" bind race that flaked #370's boot.
+          adb start-server || true
+
           booted=0
           for attempt in 1 2; do boot_emulator "$attempt" && { booted=1; break; }; done
           [ "$booted" = "1" ] || { echo "::error::API 37 preview emulator failed to boot after 2 attempts"; exit 1; }
 
           adb shell input keyevent 82 || true
-          ./gradlew connectedDebugAndroidTest --stacktrace
+          # Run only this matrix leg's shard. AndroidJUnitRunner hashes each test name into one of
+          # numShards buckets and runs only shardIndex's bucket. The args flow through AGP's
+          # -Pandroid.testInstrumentationRunnerArguments.* channel (the same one local_instrumented.py
+          # uses for .class=…) — no GMD, no orchestrator, no Gradle-side change.
+          run_shard() {
+            ./gradlew connectedDebugAndroidTest --stacktrace \
+              "-Pandroid.testInstrumentationRunnerArguments.numShards=${API37_NUM_SHARDS}" \
+              "-Pandroid.testInstrumentationRunnerArguments.shardIndex=${{ matrix.shard }}"
+          }
+          # Retry this shard's test run ONCE on failure — retry parity with the stable `e2e` matrix
+          # (which retries once, so a flaky test self-heals on API 29–36 but would otherwise wedge
+          # the required gate on API 37, e.g. #370). Per-shard, so the retry re-runs only this shard's
+          # ~T/N tests, not the whole suite. MITIGATION, NOT A FIX: a blanket retry also masks genuine
+          # regressions, so the retried-but-passed case is flagged as a ::warning:: and the real fix
+          # stays test-level. See docs/perf/api37-e2e-sharding-spike.md.
+          run_shard || { echo "::warning::API 37 shard ${{ matrix.shard }} test run failed — retrying once"; run_shard; }
 
       - name: Dump emulator log on failure
         if: failure()
@@ -446,7 +480,8 @@ jobs:
         if: always()
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with:
-          name: e2e-api37-boot-diagnostics
+          # Shard-unique name — upload-artifact@v7 errors on duplicate artifact names.
+          name: e2e-api37-boot-diagnostics-shard${{ matrix.shard }}
           path: |
             ${{ runner.temp }}/emulator.log
             ${{ runner.temp }}/logcat.txt
@@ -461,7 +496,8 @@ jobs:
         if: ${{ !cancelled() }}
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with:
-          name: e2e-test-report-api37-preview
+          # Shard-unique name — upload-artifact@v7 errors on duplicate artifact names.
+          name: e2e-test-report-api37-preview-shard${{ matrix.shard }}
           path: app/build/reports/androidTests/connected/
           if-no-files-found: warn
 

docs/perf/api37-e2e-sharding-spike.md

@@ -0,0 +1,288 @@
+<!-- SPDX-License-Identifier: GPL-3.0-or-later -->
+# API 37 preview E2E sharding — adopted at N=2
+
+**Status: ADOPTED (N=2).** `e2e-preview` is sharded across 2 parallel API 37 emulators in
+`.github/workflows/ci.yml`. This document is the design + measurement record behind that change: it
+began as a time-boxed feasibility spike (should we shard the hand-provisioned API 37 /
+`google_apis_ps16k` 16 KB-page emulator that ran the whole instrumented suite serially?) and now
+records the adopted design. `e2e-preview` was **consistently the single longest leg in CI**
+(~16.4–17.6 min on recent runs), so it set the pipeline's critical path.
+
+The analysis below was produced from CI run logs and repo config only — **no emulator was booted
+locally** (avoids the machine-freeze risk noted in the project memory and runner contention). All
+numbers come from real successful `e2e-preview` runs; the PR's own CI run validates the live 2-shard
+timings and that both shards gate `ci-passed`.
+
+## TL;DR / recommendation
+
+**ADOPTED — `e2e-preview` is sharded in CI at `N = 2`** (a `strategy.matrix.shard: [0, 1]` fan-out —
+the values are **0-based** `shardIndex`es — each shard provisioning its own API 37 emulator and running
+one half of the suite via AndroidJUnitRunner's built-in `numShards`/`shardIndex`). Projected: the leg
+drops from **~17.1 min → ~12.7 min**, cutting the whole pipeline's critical path by **~4.9 min (~28%)**
+for the cost of **+1 emulator job (~1.5× this leg's emulator-minutes)**. Two reliability fixes ship with
+it: per-shard **test-retry parity** with the stable `e2e` matrix and an **`adb start-server`** before the
+boot loop (§5.9 / §6).
+
+- **`N = 3` is a defensible stretch** (~11.2 min) but buys only ~0.7 min more of *total-CI* time, because
+  at that point the **rest of the E2E matrix (API 30, ~12.0 min) becomes the new critical path**. The
+  extra 0.7 min costs a second added emulator and +50% more preview-boot flake surface.
+- **Do NOT exceed `N = 3`.** Below ~12 min the API 29–36 matrix caps the pipeline; further API 37 shards
+  burn emulator-minutes for zero wall-clock gain.
+- **Keep local preflight single-emulator.** `api37_e2e.py` / `local_instrumented.py` must stay one cold
+  boot — a dev box has one free hypervisor (VT-x/WHPX) and the project memory explicitly warns parallel
+  emulators can freeze the machine. Sharding is a **CI-only** change.
+- The mechanism is already proven in-repo: `local_instrumented.py` passes
+  `-Pandroid.testInstrumentationRunnerArguments.class=…` to the same `connectedDebugAndroidTest` task,
+  so `numShards`/`shardIndex` args flow the identical way — **no GMD, no orchestrator, no Gradle change**.
+
+## 1. Current job anatomy + timing breakdown
+
+`e2e-preview` (`.github/workflows/ci.yml`) hand-provisions the emulator because API 37's only image is the
+nonstandard `system-images;android-37.0;google_apis_ps16k;x86_64` (16 KB page size), which neither
+`reactivecircus/android-emulator-runner` nor AGP's Gradle Managed Device DSL can build. It installs the
+SDK + image directly (`sdkmanager`), creates the AVD (`avdmanager`), cold-boots headless with a
+two-attempt retry loop, then runs `./gradlew connectedDebugAndroidTest`. `api37_e2e.py` mirrors this
+locally (identical image/flags except `-gpu auto-no-window` vs CI's `-gpu swiftshader_indirect`).
+
+### Step timing — reference run `28811804353` (branch `fix-359-sqlcipher-16kb`, **success**, total **17.6 min**)
+
+| Phase | Step(s) | Time | Re-paid per shard? |
+|-------|---------|-----:|:------------------:|
+| Job/checkout/JDK/Gradle/KVM setup | 1–6 | ~0.4 min | yes |
+| Restore image cache + `sdkmanager` install (cache **hit**) | 7–8 | ~0.7 min | yes |
+| `avdmanager create avd` | 9 | ~0.03 min | yes |
+| **Emulator cold boot** (1 failed attempt + retry) | 10a | **~1.1 min** | yes |
+| **Gradle daemon start + configure + dependency resolution** | 10b | **~4.1 min** | yes |
+| **Compile debug app + androidTest APK + install** | 10c | **~2.0 min** | yes (per runner) |
+| **Test execution** (`Starting 263 tests` → `Finished 263 tests`) | 10d | **~8.75 min** | **NO — parallelizable** |
+| Teardown, upload report/diagnostics, post-cache/gradle | 12–29 | ~0.4 min | yes |
+
+Step 10 ("Boot emulator and run E2E", 16.05 min) bundles boot + build + test into one shell step; the
+sub-splits above were recovered from the step's own timestamped log markers
+(`##[group]Start API 37 emulator`, `Starting N tests on emulator-5554`, `Finished N tests`, `BUILD SUCCESSFUL`).
+
+### The boot-vs-test ratio (the crux)
+
+Sharding only parallelizes **test execution**; every shard re-pays boot **and** the whole fixed prologue
+(setup + boot + Gradle daemon/config + compile + install + teardown). The fixed cost is **not just boot**
+(~1.1 min) — it is dominated by the ~6 min Gradle daemon-start/configure/compile block. Splitting the job:
+
+- **`T` (parallelizable test execution) ≈ 8.8 min**
+- **`B` (fixed, re-paid by every shard) ≈ 8.3 min**
+
+i.e. **B ≈ T** — roughly a 50/50 split. That is the single most important number in this analysis: because
+the fixed overhead is about as large as the test time, sharding has a **hard floor of ~8.3 min** no matter
+how many shards you add.
+
+### Cross-run confirmation (test execution is stable, not a one-off)
+
+| Run | Branch | Total leg | Test-exec window | Boot attempts |
+|-----|--------|----------:|-----------------:|:-------------:|
+| `28811804353` | fix-359-sqlcipher-16kb | 17.6 min | ~8.75 min (263 tests) | 2 |
+| `28763651551` | feat-125-imap-connection-reuse | 16.9 min | ~9.2 min (262 tests) | 1 |
+| `28760208017` | fix-359-sqlcipher-16kb | 16.4 min | ~8.4 min (261 tests) | 2 |
+
+Test execution is a tight **8.4–9.2 min**; the leg total **16.4–17.6 min**. Run-to-run variance (~±0.6 min)
+is mostly the boot retry (1 vs 2 attempts costs ~1 min) — reinforcing that boot flake, not test time, drives
+the jitter.
+
+## 2. Instrumented-test inventory
+
+- **~263 instrumented `@Test` methods across 59 test classes** (`app/src/androidTest/…`; the runner
+  reports "Starting 263 tests"). Confirmed by static count (`@Test` occurrences) and the CI log.
+- Distribution is **class-skewed**: a few heavy classes dominate — `MessageDaoTest` (17),
+  `MigrationTest` (15), `MailboxScreenTest` (13), `AccountDataMigratorTest` (8), several DAO/`*Database`
+  classes (7 each) — while most classes have 1–6.
+- The suite is a **mix** of Room/DAO/migration DB tests (CPU + disk on `google_apis_ps16k`) and Compose UI
+  tests (Espresso/`createComposeRule`). No single test is a multi-minute outlier; 263 tests / ~8.8 min ≈
+  **~2 s/test average**, so runtime is spread reasonably evenly and **count-based sharding will be
+  approximately runtime-balanced** at N = 2–3.
+
+At ~263 tests, **N = 2 (~131/shard) or N = 3 (~88/shard)** are the sensible shard counts. Beyond that the
+per-shard test time shrinks below the fixed boot/build cost and the split stops paying off (see §4).
+
+## 3. Sharding approaches
+
+### 3a. AndroidJUnitRunner built-in sharding across a GHA matrix — RECOMMENDED
+
+`AndroidJUnitRunner` has first-class sharding: it hashes each test name into one of `numShards` buckets and
+runs only `shardIndex`'s bucket. AGP surfaces runner args through Gradle properties, so on the **existing
+hand-provisioned `connectedDebugAndroidTest`** (no GMD required) each shard runs:
+
+```bash
+./gradlew connectedDebugAndroidTest \
+  -Pandroid.testInstrumentationRunnerArguments.numShards=2 \
+  -Pandroid.testInstrumentationRunnerArguments.shardIndex=${{ matrix.shard }}
+```
+
+`shardIndex` is **0-based** (a bucket in `0..numShards-1`), so the matrix values are `[0, 1]`, **not**
+`[1, 2]` — `shardIndex=numShards` would run an empty bucket and silently drop half the suite.
+Wrapped in a `strategy.matrix.shard: [0, 1]` (or `[0, 1, 2]`) fan-out where each matrix leg provisions its
+**own** API 37 emulator and runs one shard. This is the same fan-in pattern the repo **already uses** for
+the `e2e` (API 29–36) matrix, and the same runner-arg channel `local_instrumented.py` already uses for
+`…arguments.class=` — so it is a known-good mechanism here, needs **no orchestrator, no Gradle-side change,
+and no branch-protection change** (see §5).
+
+**Confirmed compatible with the hand-provisioned 16 KB job:** runner-arg sharding is a property of
+`AndroidJUnitRunner` + the AGP `connected*AndroidTest` task, entirely independent of *how* the AVD was
+created. GMD is irrelevant. Count-based, deterministic, and stable across runs.
+
+### 3b. Alternatives (rejected)
+
+- **Manual test-class partitioning** (`-Pandroid.testInstrumentationRunnerArguments.class=A,B,C` per shard,
+  or `package`/annotation buckets): lets you hand-balance by runtime, but is **brittle** — every new test
+  class must be manually assigned or it silently runs in no shard (coverage gap) or a wrong one. Not worth
+  it for a ~0.5 min balance gain over hashing.
+- **Android Test Orchestrator** (`clearPackageData`): solves per-test process isolation, **not** wall-clock;
+  it actually *slows* a run (fresh process per test). Orthogonal to sharding — skip.
+- **`maxParallelForks` (in-JVM):** N/A — that is a *JVM unit-test* knob (and the very thing #258 rejected
+  because GreenMail binds fixed ports). Instrumented tests run on the device, not forked JVMs.
+
+## 4. Expected wall-clock gain vs. CI cost
+
+Model: an N-shard leg ≈ `B + T/N` (parallel), costing ≈ `N × (B + T/N)` emulator-minutes. Using the measured
+**B ≈ 8.3 min, T ≈ 8.8 min**:
+
+| N | Leg time `B + T/N` | Δ vs today | Emulator-minutes `N × leg` | Cost multiple |
+|:-:|-------------------:|-----------:|---------------------------:|:-------------:|
+| 1 (today) | **17.1 min** | — | 17.1 | 1.0× |
+| **2** | **12.7 min** | **−4.4 min** | 25.4 | 1.5× |
+| 3 | 11.2 min | −5.9 min | 33.7 | 2.0× |
+| 4 | 10.5 min | −6.6 min | 42.0 | 2.5× |
+| ∞ | 8.3 min (floor = B) | −8.8 min | — | — |
+
+But the **total-CI critical path** is `max(all jobs)`, and the API 29–36 E2E matrix already runs **~8.3–12.0
+min (API 30 ≈ 12.0 is its ceiling)**. So the API 37 leg only helps the pipeline until it drops to that wall:
+
+| N | API 37 leg | **Total-CI critical path** = max(API 37, API 30≈12.0) | Total CI saved |
+|:-:|-----------:|:------------------------------------------------------:|---------------:|
+| 1 | 17.1 | **17.6** (API 37) | — |
+| **2** | 12.7 | **12.7** (API 37, just above the matrix wall) | **~4.9 min (~28%)** |
+| 3 | 11.2 | **12.0** (now **API 30** caps it) | ~5.6 min (~32%) |
+| 4 | 10.5 | 12.0 (API 30) — **no gain over N=3** | ~5.6 min |
+
+**Point of diminishing returns.** Two effects compound:
+
+1. **B ≈ T** gives a hard ~8.3 min floor for the leg itself (N = ∞).
+2. Long before that, the **~12.0 min API 30 matrix wall** caps *total* CI. N = 2 lands the leg at 12.7
+   (~0.7 above the wall); N = 3 tucks it under the wall (API 30 takes over). **N = 3 is the last count that
+   changes anything**; N ≥ 4 is pure waste unless the API 29–36 matrix is *also* sharded (a separate,
+   larger effort).
+
+**Verdict:** **N = 2 captures ~4.9 of the ~5.6 min theoretically achievable** — the overwhelming majority
+— for the least cost and flake. N = 3 collects the last ~0.7 min at +1 more preview emulator.
+
+## 5. Risks / gotchas
+
+1. **Flake multiplication (top code-risk).** `e2e-preview` is already the flakiest leg (16 KB preview image;
+   boot/snapshot races; the `input keyevent 82` timing hazard; infra-vs-code ambiguity). N shards = N
+   independent cold boots per pipeline, so `P(all shards boot)` = `p^N`. Mitigation is **built in**: each
+   shard inherits the job's existing **two-attempt boot loop** (the memory note pegs a single un-retried
+   boot race at ~2% → ~0.04% after two attempts), keeping aggregate boot success ~99.9% even at N = 3. Keep
+   the retry loop per shard; do **not** collapse it to save time.
+2. **Runner concurrency (top practical-risk).** The pipeline already fans out 8 E2E matrix jobs +
+   `e2e-preview` + 3 lighter jobs (~12 concurrent). N = 2/3 pushes peak to ~13/14 concurrent runners. **If
+   the account's concurrent-job limit is hit, shards queue serially and the entire wall-clock gain
+   evaporates** (or inverts). The project already actively manages runner allocation/priority (P0–P9), so
+   this is a real, tracked constraint — confirm headroom before enabling, and give the preview shards the
+   same priority as the rest of the E2E matrix.
+3. **`ci-passed` gate must require ALL shards — but needs no branch-protection edit.** Converting
+   `e2e-preview` to a matrix makes GHA roll all shard legs under the one `e2e-preview` entry already in
+   `ci-passed.needs`; the gate's `contains(needs.*.result, 'failure' | 'cancelled' | 'skipped')` check
+   fails if **any** shard fails (a matrix job's `result` is `failure` when any leg fails). Branch protection
+   only requires the single `CI passed` context, so **no protected-check list change** — same as the
+   existing `e2e` matrix. Verified against the current gate wiring.
+4. **Artifact-name collision.** Each shard uploads its own `app/build/reports/androidTests/connected/` HTML
+   report; `actions/upload-artifact@v7` **errors on duplicate names**. The report name must gain the shard
+   index (e.g. `e2e-test-report-api37-preview-shard${{ matrix.shard }}`), mirroring the API-level suffix the
+   `e2e` matrix already uses. Same for the boot-diagnostics artifact.
+5. **No cross-shard coverage merge needed (a simplifier vs #258).** #258's blocker was JaCoCo aggregation
+   across shards. Here, **instrumented/E2E coverage is explicitly out of scope** (`app/build.gradle.kts`
+   JaCoCo scoping + issue #192), so there is nothing to merge — shard results only need to fan into the pass/
+   fail gate.
+6. **Cold-cache thundering herd.** The ~1 GB preview image is cached (`actions/cache`, keyed identically for
+   all shards). On a **warm** cache all shards get a fast hit (steady state). On a **cold** cache (first run
+   after a key change), all N shards download ~1 GB concurrently — a rare N× bandwidth spike, self-healing
+   on the next run. Acceptable; no change needed.
+7. **Shard balance.** Count-based hashing is only *approximately* runtime-balanced; a shard that draws
+   `MigrationTest` + the DAO-heavy classes may run marginally longer. Tolerable at N = 2–3 (§2); revisit only
+   if a shard consistently lags.
+8. **`concurrency: cancel-in-progress` churn.** Unchanged semantics (all shards share the PR concurrency
+   group, cancelled together on a new push), but there are now more in-flight jobs to cancel and re-trigger
+   on each push — a small addition to the known CI merge-cascade thrash. No new failure mode.
+9. **Retry-parity gap — a *pre-existing* reliability asymmetry sharding must preserve (surfaced by #370).**
+   The stable `e2e` matrix retries its test run **once** on failure (`ci.yml`, the two paired
+   `reactivecircus/android-emulator-runner` "Run E2E tests" + "…(retry…)" steps), so a genuinely flaky test
+   self-heals on API 29–36. **`e2e-preview` runs `connectedDebugAndroidTest` exactly once — no retry**, so
+   the *same* flaky test **wedges the required gate on API 37**. This is exactly what flaked #370 (a real
+   `SignaturesScreenTest` Room/`viewModelScope` teardown race, being fixed at the test level separately) —
+   not a boot/timing issue. **Interaction with sharding (why it belongs here):** unlike **boot** flake,
+   which sharding *multiplies* by N (risk #1), per-**test** flake is **not** amplified — every test still
+   runs exactly once across the whole matrix, so a single flaky test lives in exactly one shard. But it
+   still fails a *required* leg, so the gate needs a retry regardless of shard count. The right design is a
+   **per-shard** retry: each shard retries only its own bucket, costing **`B + T/N`** (one shard's leg),
+   strictly cheaper than the unsharded whole-suite retry (`B + T`). So sharding makes flake-recovery both
+   still-necessary and *cheaper*. **Framing: mitigation, not a fix** — a blanket retry also masks genuine
+   regressions, so surface the retried-but-passed case as a `::warning::` and keep the real fix test-level
+   (as #370 does). See §6 for the recommendation and the implementation.
+
+## 6. Recommendation (adopted)
+
+**Worth doing: yes — done.** `e2e-preview` is the pipeline's critical path, so unlike #258 (unit tests, which
+finish ~8.5 min *inside* the E2E-bounded gate and were correctly closed as not-worthwhile) sharding this leg
+**directly** shortens total CI.
+
+- **Adopted `N = 2`** via a `strategy.matrix.shard: [0, 1]` fan-out using AndroidJUnitRunner
+  `numShards`/`shardIndex` (§3a). Expected total-CI critical path **~17.6 → ~12.7 min (~28% off)** for **+1
+  emulator job**. Each shard keeps the two-attempt boot loop; the report/diagnostics artifact names carry the
+  shard index; `e2e-preview` stays a single entry in `ci-passed.needs` (matrix fan-in keeps the single gate —
+  both shards must pass; §5.3). `fail-fast: false` so a failing shard doesn't cancel its sibling.
+- **`N = 3` not adopted** — reserved for if the last ~0.7 min ever matters and runner headroom is comfortable;
+  it moves the critical path onto the API 30 matrix leg (~12.0 min). **Never `N ≥ 4`** without also sharding
+  the API 29–36 matrix.
+- **Local preflight stays single-emulator** — `api37_e2e.py` and `local_instrumented.py` keep their single
+  hand-provisioned boot (not sharded). One free hypervisor per dev box; parallel local emulators risk freezing
+  the machine (project memory). The sharding divergence is intentional and is documented in the job comment.
+
+Two **reliability** fixes ship with sharding (from the #370 root-cause; §5 item 9) — together they make the
+API 37 leg *faster **and** more reliable*:
+
+- **Retry parity — shipped (CI).** `e2e-preview` now gives each shard the **same single test-run retry** the
+  `e2e` matrix has, as a **per-shard** retry (re-runs only that shard's tests, not the whole suite). **This is
+  a mitigation, not a fix:** a blanket retry masks genuine regressions, so the retried-but-passed case is
+  flagged as a `::warning::` and real flakes stay fixed at the test level (as #370 did). The retry count is
+  **one**, not more. *Sharding interaction:* a per-shard retry is the natural fit and is cheaper than an
+  unsharded retry — see §5 item 9. **Local mirror deferred:** mirroring the same single test-retry into
+  `api37_e2e.py`'s `connectedDebugAndroidTest` step (for CI/local lockstep) is a small follow-up, kept out of
+  this change because it can't be validated without booting a local emulator (which this change deliberately
+  does not do). `api37_e2e.py` already matches CI on the boot-retry loop and `adb start-server`.
+- **`adb start-server` before the boot loop — shipped (CI).** Mirrors `api37_e2e.py`'s `wait_for_boot` (which
+  already does this), avoiding the attempt-1 adb-daemon "Address already in use" bind race seen in #370
+  (self-recovered there; free hardening).
+
+### Relationship to #258 (unit-test sharding)
+
+#258 was closed **not-worthwhile** because the unit-test job (~6.5 min) isn't on the critical path (E2E
+dominates), so sharding it saved ~0 wall-clock, and `maxParallelForks` collided on GreenMail's fixed ports.
+**Reused pattern:** *measure the critical path first, only shard what's on it.* API 37 preview **is** on it,
+so the conclusion flips to GO. **Distinct from #258:** different layer (on-device instrumented vs JVM),
+different mechanism (`numShards`/`shardIndex` matrix vs `maxParallelForks`), and **no coverage-merge
+constraint** (instrumented coverage is out of scope), which makes this strictly simpler to land.
+
+## Implementation
+
+This change converts `e2e-preview` to an `N = 2` shard matrix (`strategy.matrix.shard: [0, 1]`,
+`fail-fast: false`), each leg hand-provisioning its own API 37 emulator and running one shard via
+`-Pandroid.testInstrumentationRunnerArguments.numShards=2 -Pandroid.testInstrumentationRunnerArguments.shardIndex=<0|1>`.
+Each uploaded artifact name carries the shard index (`…-shard${{ matrix.shard }}`) — `upload-artifact@v7`
+errors on duplicate names. It also folds in the two §6 reliability fixes on the CI side: a **per-shard single
+test retry** (retry parity with the `e2e` matrix, flagged `::warning::` on the retried-but-passed case) and
+an **`adb start-server` before the boot loop**. The matching `api37_e2e.py` local retry mirror is a
+documented follow-up (§6) — not bundled here because it can't be validated without booting a local emulator.
+
+**Validation.** Because a PR runs the workflow *from its own branch*, **this PR's own CI run is the
+validation** — both shards must go green and the real per-shard wall-clock is readable straight off that run
+(this is a CI-infra change, so a green pipeline is the "test", per #258's DoD note). Both shards fan into the
+single `e2e-preview` entry in `ci-passed.needs`; GHA's matrix aggregation makes `e2e-preview`'s result
+`failure` if either shard fails, so `ci-passed` (the branch-protection-required "CI passed" context) stays
+red unless **both** shards pass — no branch-protection change needed (§5.3).