Anyone can write a limit.

Nobody wants every deployment to own the lifecycle of authority: who holds it, what is left, who changed it, how to take it back.

Twenty agents, twenty balances	Each agent draws on its own card. One agent can never spend another's authority.
Two runs of one agent spend at once	Draws on that agent's card are locked. The second is refused if the first leaves too little.
A request is retried	Counted once.
The balance cannot be read	Refused, never treated as zero.

Proof that this is infrastructure worth centralizing, not the reason to buy it.