This stack layer makes app-server thread event delivery connection-aware
so resumed/attached threads only emit notifications and approval prompts
to subscribed connections.
- Added per-thread subscription tracking in `ThreadState`
(`subscribed_connections`) and mapped subscription ids to `(thread_id,
connection_id)`.
- Updated listener lifecycle so removing a subscription or closing a
connection only removes that connection from the thread’s subscriber
set; listener shutdown now happens when the last subscriber is gone.
- Added `connection_closed(connection_id)` plumbing (`lib.rs` ->
`message_processor.rs` -> `codex_message_processor.rs`) so disconnect
cleanup happens immediately.
- Scoped bespoke event handling outputs through `TargetedOutgoing` to
send requests/notifications only to subscribed connections.
- Kept existing threadresume behavior while aligning with the latest
split-loop transport structure.
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