Function memoryRateLimiter

Builds an in-process RateLimiter counting fixed windows in a Map. It exposes the same observable contract as the Redis adapter and serves as a usable single-process limiter when no Redis is wired.

Each key gets limit requests per windowMs, measured against the injected clock; a denial reports how long until the window resets. An expired window is replaced only when its key is next seen, so the map retains every key it has ever counted.