RPC atomic snapshot cursors, UI-answer receipts, and dialog observation #4266

Summary

Add atomic snapshot cursors, UI-answer receipts, and opt-in dialog observation to RPC.

Expected

• get_snapshot captures state and bounded newest history synchronously with a connection-local snapshot_marker.
• answerId receipts distinguish applied, replay, already_answered, and unknown outcomes with a bounded in-memory session ledger.
• dialogs:true mirrored subscriptions observe pending and settled UI requests without attachment or command replies.
• Terminal control endpoints answer snapshots. Existing state fields and targeted dialog exclusions remain intact.

Scope

RPC request/response types, session projection, FIFO routing, dialog lifecycle hooks, tests, and protocol documentation.

Related

Implementation plan todo 46. No matching prior issue or PR found in snapshot-marker and UI-answer receipt searches.