test: make Android lifecycle race coverage deterministic

[BeforeLights]

Summary

• start sign-out undispatched so the lifecycle test must contend on the per-scope mutex
• document the automatic CodeRabbit follow-up

Verification

• ./gradlew.bat :app:testDebugUnitTest --offline
• git diff --check

Review-flow note

This is a focused promotion-gate follow-up for the automatic CodeRabbit review of PR #14 after PR #17. It is intentionally a small exception to the normal 30–70 commit batching rule. CodeRabbit is not being invoked on this dev PR.

[coderabbitai]

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