ci: add theme instrumentation gate and theming backfill (R571)

[ryacub]

Ticket

• ID: R571
• Priority: P2
• Risk Tier: T1
• GitHub Issue: Closes R569: theming test governance — CI instrumentation for custom theme flows #571

Objective

• Add a dedicated CI gate for theme-related instrumentation coverage and complete theming-related changelog backfill notes without widening functional scope.

Scope

• Add theme instrumentation workflow gated by theming-related paths.
• Add new instrumentation tests for custom theme accent persistence/reset/fallback.
• Update existing theme appearance android test callsite to match current widget params.
• Backfill missing Unreleased changelog note and add a short README note for the new CI gate.

Non-goals

• No theme engine refactor.
• No runtime behavior changes outside theming test/CI coverage.
• No broad androidTest architecture changes.

Files Changed

• .github/workflows/theme_instrumentation_pr.yml
• app/src/androidTest/java/eu/kanade/presentation/theme/ThemePreferencesInstrumentationTest.kt
• app/src/androidTest/java/eu/kanade/presentation/more/settings/widget/ThemeAppearanceFlowAndroidTest.kt
• CHANGELOG.md
• README.md

Verification

• Commands run:
  • ./gradlew spotlessCheck
  • ./gradlew :app:compileDebugKotlin
  • ./gradlew :app:testDebugUnitTest --tests "*ThemeModeTest*" --tests "*UiPreferencesTest*" --tests "*TachiyomiThemeRoutingTest*"
  • ./gradlew :app:assembleDebugAndroidTest
• Results:
  • All commands above passed locally.
• Not tested:
  • ./gradlew :app:connectedDebugAndroidTest (no connected emulator/device available locally at execution time; covered by PR CI workflow).

Risk

• Main risk is instrumentation test flakiness on emulator CI. Mitigation: path-scoped trigger and failure artifact upload (reports + logcat).

SLO Impact

• No runtime SLO impact. CI-only/test-only hardening.

Rollback

• Revert this PR commit to remove the workflow/tests/docs additions.

Release Notes

• Added a dedicated theme instrumentation PR gate and additional Android instrumentation coverage for custom theme accent persistence/reset/fallback.

Checklist

• Naming conventions followed (use "Rayniyomi" in user-facing text, see naming conventions)
• Branch rebased on latest main and verification re-run (see rebase policy)
• New coroutine scopes have documented owner and cancellation path
• No new runBlocking on UI/main thread paths

Definition of Done (DoD)

• Acceptance criteria met and non-goals respected
• Verification matrix completed (Risk Tier: T1)
• Self-review completed
• Rebased on latest main and revalidated (mandatory for merge)
• Sprint board updated (branch, commit, checks, PR link)
• Release notes drafted (if applicable)

Fork Compliance Checklist (for new forks only)

If you are creating a new fork of this project, ensure the following:

• App Identity: Changed app name from "Aniyomi" to your fork name
• App Icon: Replaced launcher icon assets with fork-specific icons
• Application ID: Changed applicationId in app/build.gradle.kts from xyz.rayniyomi to your unique ID
• Update Checker: Configured AppUpdateChecker.kt to point to your fork's repository
• Analytics: Either disabled Firebase Analytics or configured with your own google-services.json
• Crash Reporting: Either disabled ACRA crash reporting or configured with your own endpoint credentials

See CONTRIBUTING.md Forks section for details.