[codex] Fix scratch storage promotion and add coverage gate

[stozo04]

Summary

• Move active capture/import scratch files from cacheDir/scratch to filesDir/scratch so user-visible in-flight videos are not stored as cache-evictable data.
• Keep reverse-preview derived cache files under cacheDir/scratch/reversed, and prune new active scratch, legacy cache scratch, and reverse cache paths.
• Add a ViewModel save-in-progress guard to ignore duplicate Save taps while promotion/enqueue/render is already active.
• Add Crashlytics source_exists keys so future events distinguish missing files from real zero-byte files.
• Add JaCoCo JVM/Robolectric coverage reporting plus a 90% ViewModel coverage gate wired into :app:check.

Root Cause

Crashlytics showed promoteScratchToRaw() failing before render enqueue because Kotlin File.copyTo() could not find the scratch source file under cacheDir/scratch/raw_<uuid>.mp4. This is not the intentional post-render cleanup path: the worker only discards scratch and deletes the persisted raw after render succeeds. The source file was missing before WorkManager render enqueue.

The artifact proves the active scratch was missing at promotion time. Cache eviction is the storage-design risk this PR removes by moving active user work out of cacheDir; the fix also keeps explicit app deletion paths auditable.

Validation

• :app:testDebugUnitTest passed.
• :app:jacocoDebugUnitTestReport passed.
• :app:jacocoDebugViewModelCoverageVerification passed.
• :app:check passed.
• git diff --check passed.

Coverage from the generated JaCoCo XML:

• Full app instruction coverage: 15.89%.
• Full app line coverage: 21.61%.
• ViewModel gate instruction coverage: 90.73%.
• ViewModel gate line coverage: 91.90%.

Known Gaps

• Instrumented tests were not run because the only local Android target was emulator-5554 offline.
• Whole-app 90% coverage is not gated yet because current JVM tests do not cover large UI/media/hardware paths. The full report is generated so that debt is visible while the ViewModel state-machine layer is enforced at 90%.