Single-node ETS implementation of Attesto.CNonceStore.

Nonces are random 128-bit base64url strings held in a public ETS table owned by a GenServer that sweeps expired entries. Validation is non-consuming, so a c_nonce may be used by several proofs in one batch.

This is a per-node store; a multi-node deployment MUST back Attesto.CNonceStore with a shared store. Like the other ETS stores it refuses to boot on a clustered BEAM unless multi_node_acknowledged?: true.

Start options: :sweep_interval_ms (default 30_000), :multi_node_acknowledged? (default false).