Single-node ETS implementation of Attesto.StatusListStore.

Allocation and status updates are serialized through the owning GenServer, making concurrent allocations for one URI atomic. Each URI is stored as one ETS row containing its next index and current status overrides.

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

Start option: :multi_node_acknowledged? (default false).