refactor([issue-4283]): share mic capture re-entrancy guard

[atomantic]

Closes #4283

Summary

• Extract the pending microphone permission and generation guard into useAsyncCaptureGuard.
• Reuse it from sing-to-score and sing-to-verify without changing capture teardown behavior.
• Export and document the shared hook, with focused guard tests.

Test plan

• npm test -- --run useAsyncCaptureGuard.test.js useSingToScore.test.js useSingToVerify.test.js
• npm test (client): 650 files and 7,935 tests passed.
• npm run lint (client)
• npm run build (client)
• git diff --check