AuthenticationNonceRepository

Nonces of accepted authenticated requests, per device, for replay protection (docs/server-authentication.md). Entries only need to live as long as their request timestamp is inside the server's validity window.

The repository keeps a monotonic prune watermark (S1, finding F8): the largest claim pruneBefore it has ever applied. Every entry whose timestamp is at or after the watermark is retained, and every claim whose timestamp is before it is refused, because its nonce may already have been pruned. So no interleaving of claims whose callers read their clocks at different times can prune a nonce and then accept it again.