Stop the staging fixture losing a race with the app-start sweep (#159)

`the sweep tolerates a staging path that is not a directory` failed once on run
33069641674, against 468 tests that pass on this machine including under
`--rerun-tasks`:

    java.io.FileNotFoundException at OutputPublisherStagingTest.kt:112
    468 tests completed, 1 failed

Line 112 was `writeBytes` immediately after `deleteRecursively()`.
`FileOutputStream` answers `FileNotFoundException` for an existing directory, so
something had recreated the path inside that window. That something is
`LibreMediaConverterApp.onCreate`, which ends with

    appScope.launch { OutputPublisher(...).sweepStaging() }

on `Dispatchers.IO`, and `sweepStaging` reads `stagingDir`, whose getter calls
`mkdirs()`. Robolectric builds the application for every test that asks for one,
so that background `mkdirs()` is in flight across the whole suite on a thread the
paused main looper does not control and no test awaits.

Retrying closes the window rather than narrowing it, because the race is not
symmetric: `mkdirs()` fails on an existing regular file, so the invariant only has
to survive being *established*. Once a write lands, nothing in the suite can turn
this path back into a directory -- which is also why the new assertion that the
sweep left a file behind is worth making.

The `check()` matters as much as the loop. The next failure here should say
"something recreated conversions/ as a directory", not `FileNotFoundException at
line 112` -- that is the difference between a flake someone reads and a flake
someone re-runs.

The wider problem is #159 and is deliberately not fixed here: `AppStartSweepTest`,
`JobSnapshotsTest` and `SpaceArithmeticTest` all name the same path, and the real
answer is an injectable scope rather than a retry loop in every staging test.
#159's done-when is that this loop can be deleted.

Mutation: `listFiles() ?: return` -> `listFiles()!!` reddens exactly this test.
Gate green: assembleDebug, testDebugUnitTest, compileDebugAndroidTestKotlin,
ktlintCheck, detekt, lintDebug.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src/test/java/org/libremediaconverter/convert/OutputPublisherStagingTest.kt

@@ -108,10 +108,46 @@ class OutputPublisherStagingTest {
         // cannot be a directory makes listFiles() answer null, and a sweep that dereferenced that
         // would take the app down on a launch rather than on a conversion -- AppStartSweepTest is
         // where this runs from.
-        File(cacheDir, "conversions").deleteRecursively()
-        File(cacheDir, "conversions").writeBytes(ByteArray(8))
+        val stagingPath = stagingPathAsRegularFile()
 
         publisher.sweepStaging()
+
+        assertTrue("the sweep must not have replaced the fixture", stagingPath.isFile)
+    }
+
+    /**
+     * Makes `cacheDir/conversions` a regular file, which is the whole precondition of the test
+     * above -- and does it in a loop, because a single delete-then-write loses a race that CI
+     * caught and this machine does not reproduce.
+     *
+     * `LibreMediaConverterApp.onCreate` ends with
+     * `appScope.launch { OutputPublisher(...).sweepStaging() }` on `Dispatchers.IO`, and
+     * `sweepStaging` reads `stagingDir`, whose getter calls `mkdirs()`. Robolectric instantiates
+     * the application for every test that asks for one, so that background `mkdirs()` is in flight
+     * across the whole suite, on a thread the paused main looper does not control. Between deleting
+     * this path and writing it there is a window where the path does not exist and that `mkdirs()`
+     * can win, which is `FileNotFoundException: ... (Is a directory)` out of `writeBytes` -- run
+     * 33069641674 on #149, once, against 468 tests that pass here.
+     *
+     * Retrying closes it rather than narrowing it, because the race is not symmetric: `mkdirs()`
+     * fails on an existing regular file, so the invariant only has to survive being *established*.
+     * Once a write lands, nothing in the suite can turn this back into a directory.
+     *
+     * The wider problem -- application-scope IO work racing every Robolectric test that shares
+     * `cacheDir` -- is #159, and is deliberately not fixed here.
+     */
+    private fun stagingPathAsRegularFile(): File {
+        val stagingPath = File(cacheDir, "conversions")
+        repeat(FIXTURE_ATTEMPTS) {
+            if (stagingPath.isFile) return stagingPath
+            stagingPath.deleteRecursively()
+            runCatching { stagingPath.writeBytes(ByteArray(FIXTURE_BYTES)) }
+        }
+        check(stagingPath.isFile) {
+            "the fixture needs $stagingPath to be a regular file and it is a directory; " +
+                "something recreated it $FIXTURE_ATTEMPTS times -- see #159"
+        }
+        return stagingPath
     }
 
     @Test
@@ -125,4 +161,10 @@ class OutputPublisherStagingTest {
 
         assertFalse("a file with no parent is not in staging", publisher.discardStaged(parentless))
     }
+
+    private companion object {
+        /** Enough to outlast a burst of application-scope sweeps; one attempt is what CI lost. */
+        const val FIXTURE_ATTEMPTS = 50
+        const val FIXTURE_BYTES = 8
+    }
 }