Move time by hand

Implement FakeClock(start=0.0) and a RateLimiter(rate, window, clock) that uses it: - FakeClock: now() returns the current value; advance(seconds) moves it forward (negative advance asserts) - RateLimiter: allow() permits at most rate events in any window of window seconds, using ONLY the injected clock (never time.*); events older than the window stop counting Determinism under test: the suite must be able to replay any scenario exactly.

Difficulty: advanced