Make mobile Android validation risk-based

[stef-k]

Summary

• keep the short unit/core workflow automatic for ordinary code PRs
• make the Android Release workflow manual and risk-based
• document the exact MAUI, platform, dependency, and release boundaries requiring Android validation
• add concurrency cancellation to both workflows
• avoid documentation, tests-only, Core-only, superseded, and post-merge Android builds
• retain the manual Android artifact upload unchanged

Validation

• tests passed on the initial PR head
• automatic Android run cancelled after the policy was narrowed
• manual-trigger and concurrency source assertions
• git diff --check

Artifact clarification

The temporary one-day Actions artifact remains available from manual Android builds. It is separate from the APK attached to formal GitHub Releases.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
WayfarerMobile.Core	80%	69%	644	✔
Summary	80% (838 / 1042)	69% (363 / 529)	644	✔