test: harden CI timing checks

[Helweg]

Summary

• expose Git watcher readiness and include it in combined watcher readiness
• replace timing sleeps and single-shot filesystem writes with readiness-based, retry-until-observed integration assertions
• make the 10k-node community performance test use a warm median while retaining cold and outlier ceilings

Validation

• watcher suites passed 5 consecutive runs under concurrent Rust timing load
• Rust 10k-node timing test passed 5 consecutive runs
• npm run build
• npm run typecheck
• npm run lint
• npm run test:run (78 files, 1330 tests)