Build a token-bucket limiter with exact semantics

Create src/rate.js exporting class TokenBucket { constructor(rate, capacity, now) }. The bucket starts FULL. take(n=1) removes n tokens and returns true if there were enough, else false and does NOT modify the bucket. Tokens refill at `rate` per second continuously. `now` defaults to () => Date.now(). DO NOT use setTimeout. The test in test.js covers full, partial, empty, and refill.