Clears an instance's expired time-window slots shortly after each epoch boundary.

For each window, a timer fires just after every epoch starts and clears the slot the next epoch will use (see Limen.State.Window.rotate/3). A whole slot is dropped at once, so no per-entry expiry scan ever runs.

Every minute it also starts new distinct-counting generations, and every second it publishes a fresh estimate of active client prefixes, so the request path never has to scan a HyperLogLog.