pub const DURABLE_KEY_ID: u8 = 0;Expand description
The key-id byte stamped on every payload sealed with the current ZEPH_DURABLE_KEY.
seal writes this as the leading byte and open selects the current key by it. Both the
agent-loop engine and the zeph durable --reveal CLI build the cipher with this id so a sealed
blob round-trips. Rotating to a fresh key bumps the id and registers the old one as the previous
slot (XChaCha20Poly1305Cipher::with_previous).