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Request × Key-State Matrix

A (Acquire)

Key state	Server action	Response	Timing
Not registered	Register (expiry = now+lease), issue token	`A`+token+key	Immediately
Registered + expiry passed	Renew (now+lease), new token	`A`+token+key	Immediately
Registered + valid (held)	Register on wait queue, hold response	`A`+token+key or `T`+key	The moment it's acquired via release/expiry, or `T` if `wait` is exceeded

The release path is FIFO-fair: when the holder does R, it's handed directly to the waiter at the front of the queue (no re-contention, a new token).
Only the expiry path is non-FIFO. No re-entrancy (the same client doing A again on a key it already holds → goes to the wait queue).

R (Release)

Key state	Server action	Response
Registered, has waiters	Hand off directly to the waiter at the front (new token)	`R`+key
Registered, no waiters	Delete the key	`R`+key
Not registered	Nothing	`N`+key

Field Constraints