fix: make server truth end stale web streams

[sam-saffron-jarvis]

What

Make stalled response recovery converge on server truth without allowing an old idle sample to cancel a newer send:

• reconnect-loop state reconciliation failures are caught and treated as unknown, so replay keeps retrying
• a rejected resume detaches only the matching failed transport, then performs selected-session reconciliation
• sidebar status detecting “server idle, client still owns a server-issued response ID” confirms through /state
• idle /state may retire a transport only when its controller, generation, session ID, and non-empty response ID are unchanged across the state round trip
• unacknowledged POST /v1/responses ownership (currentStreamResponseId === '') is never retired by idle polling
• normal state polling remains suppressed while a healthy transport is attached; no extra active-run poll loop

Why

Session 3137 exposed the failure sharply:

• the final response was persisted at 18:40:12 AEST
• the client’s last /events connection had ended at 18:28:24
• the browser still claimed the run was active
• Stop at 18:58:29 received 404 because the response had already been done for 18m 16s
• reload showed the correct durable transcript immediately

The likely root failure was a rejected reconciliation killing the nominally infinite reconnect loop. The fix makes that loop exception-safe and adds an independent, ownership-safe server-truth path.

Race invariants

1. A stable stale transport with a server-issued response ID can be retired when /state confirms idle.
2. An idle response sampled before a newer local send cannot abort that send.
3. Sidebar idle status cannot abort a response POST before the server has issued its response ID.
4. A transient reconciliation exception cannot terminate heartbeat recovery.

Verification

• node internal/serveui/static/app_sessions_test.js
• node internal/serveui/static/app_stream_test.js
• go test ./internal/serveui/...
• go build .
• all three differential race probes from the first Opus review now pass:
  • newer run survives stale idle state
  • in-flight response POST survives sidebar idle status
  • no perpetual 1.2-second /state polling loop