docs: document local timezone mocking in tests runbook

🎯 What needs to be done

Add a runbook section in docs/TESTING.md documenting how to use TestClock and time.ts utilities to mock dates, midnight boundaries, and specific IANA timezones in unit tests.

Why this matters

PACT guarantees timezone safety across global timezones (e.g. America/New_York, Asia/Kolkata, Europe/London). Explaining how to test timezone-sensitive code helps contributors write robust unit tests.

📍 Where to work

• docs/TESTING.md
• tests/temporal-engine.test.ts (Reference existing test patterns)

🧭 Implementation guidance

1. Open docs/TESTING.md and locate Section 3 ("Test Authoring Patterns").
2. Add a sub-section titled "Timezone & Clock Mocking".
3. Provide code snippets showing how utcToLocal, localToUtc, and injected timestamp parameters work in domain tests.

✅ Acceptance criteria

• docs/TESTING.md explains timezone mocking patterns clearly.
• Examples reflect actual utilities in src/lib/time.ts.

Start in Section 3 of docs/TESTING.md, then read tests/temporal-engine.test.ts and src/lib/time.ts to verify the existing TestClock, time.ts, timezone, and injected-timestamp patterns. Document those patterns with examples for utcToLocal, localToUtc, midnight boundaries, IANA timezones, and domain tests. Done means the runbook is clear and all examples match the actual utilities.