Reserving a slot per request refused a correct password with the slots it had
filled itself: after a restart the browser replays its cached Basic header on many
parallel requests. Identical credentials from one IP now join the verify already in
flight for them (keyed on a SHA-256 digest), skipping the gates and its scrypt cost.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>