Declare build.yml's token reach in build.yml

CodeQL alert #1, the only open one on this repository:

  actions/missing-workflow-permissions, warning / medium, build.yml:23
  Actions job or workflow does not limit the permissions of the GITHUB_TOKEN.

Alerts 2, 3 and 4 were the same rule against status_check.yml and are fixed -- that file
has a top-level block. build.yml declares permissions in exactly one place, the release
job's `contents: write`, and has no top-level default, so the `test` job inherits the
repository setting.

**Nothing is over-privileged today.** The repository default is already `read`
(default_workflow_permissions: read, can_approve_pull_request_reviews: false, read from the
API rather than assumed), so the test job holds a read token now. Saying so matters: this
is hygiene, and a commit that implied it was closing a live hole would be overstating it.

What it buys is that the default CANNOT widen these jobs later without someone editing this
file. That is not invented for the occasion -- it is the argument status_check.yml already
makes, which even names this file:

  the token's reach should be readable here, and a default that widens later should not
  silently widen these jobs with it. build.yml's release job makes the opposite
  declaration for the same reason.

So the principle was decided, applied in two workflows and in one job of this one, and the
top level of build.yml was the gap.

Verified the thing that would actually break: the release job's `contents: write` still
wins. Top level is a default, not a ceiling -- parsed and printed both, test inherits
`contents: read`, release keeps `contents: write`.

Also ran the ticket's mutation, and it found something. Deleting the release job's
`contents: write` leaves actionlint green and CodeQL quiet -- a narrower permission is not
an alert -- so nothing would catch it until a tagged release failed to publish. That is a
separate gap and is filed rather than fixed here.

actionlint clean at the pinned digest. Comment and permissions only; no step, job or
trigger changes.

Closes #100.

.github/workflows/build.yml

@@ -13,6 +13,17 @@ on:
 # reference amounts to running whatever that repository contains tomorrow. This matters
 # more here than on pull requests: these jobs sign nothing today, but they do publish
 # the artifacts people install.
+# Declared here rather than inherited, for the reason status_check.yml gives for its own
+# block: the token's reach should be readable in the file that uses it, and a repository
+# default that widens later should not silently widen these jobs with it. The repository
+# default is `read` today, so this changes nothing about what runs -- it fixes what a
+# reader can know without leaving the file, and it is what CodeQL alert #1 asked for.
+#
+# The `release` job below overrides this with `contents: write`, which is how job-level
+# permissions work: this is a default, not a ceiling.
+permissions:
+  contents: read
+
 env:
   GRADLE_CACHE_PATHS: |
     ~/.gradle/caches