Start takes a free slot or overwrites the one expiring first; parallel starts carry the same request
cookies, choose the same slot and overwrite each other. Complete finds its attempt by scanning the
slots for the posted nonce; an overwritten attempt finds none and is refused like any other. The
per-request counting sweep, its byte budget and the nonce-named cookie are deleted.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>