fix(web): make microphone lifecycle deterministic

[rogerchappel]

Summary

• share one startup promise across overlapping start() calls and reuse the active session result
• serialize stop() against startup so all acquired tracks, nodes, timers, and contexts are released
• cover sequential starts, overlapping starts/stops, and restart-after-stop; document the lifecycle contract

Commits

• test(web): cover repeated and overlapping starts
• fix(web): serialize microphone lifecycle transitions
• test(web): verify restart after complete teardown
• docs(web): define idempotent lifecycle contract

Verification

• npm run check
• npm test (39 tests)
• npm run typecheck
• npm run build
• npm run package:smoke
• git diff --check origin/main..HEAD

All proposed commits were audited as authored and committed by Roger Chappel miscanalysis@gmail.com.