feat(agent): evaluate optional final snapshot delivery before termination

[ben-ranford]

Outcome

Evaluate an opt-in guarantee that a subscription emits the final valid snapshot before its session_closed terminal notification, with bounded transport behavior.

Current contract and evidence

The v1.1.0 implementation in PR #97 promises a bounded, monotonically increasing subsequence of snapshots. Terminal state removes the subscription; it does not guarantee delivery of the final snapshot. snapshotSubscription.fail clears its pending slot and the writer gives terminal messages priority. WaitForPublication in PR #78 explicitly returns ErrSessionClosed when the session closes.

Integration review of local candidate 173b633882c44f6a80a99fdf07b5dd3fa078ddc8 found that the admission-overload shutdown in PR #73 can follow a final result publication immediately. A slow subscription can consequently receive session_closed without that final snapshot. The final state remains accessible through Session.Snapshot and the replay transcript. Independent scope review classified this as a possible new delivery guarantee, not a violation of the current coalescing/terminal contract or a blocker for #45/#49/#65.

Bounded design and acceptance checks

• Decide whether final-state delivery should be optional and separately negotiated; preserve existing terminal-priority behavior for current clients.
• Define a bounded policy for slow/failed writers, cancellation, output limits and provider failures. Do not block unrelated clients or create an unbounded queue.
• If adopted, prove a final valid snapshot precedes exactly one terminal message using a controlled slow writer and an actual BindSession session-close/overload fixture.
• Keep replay, polling patch history, redaction and capability checks intact.
• Do not silently change the documented closed-session behavior of WaitForPublication as a side effect.

This is an integration-derived product hypothesis, not validated customer demand. It is intentionally outside the current v1.1.0 milestone scope.