fix(tui): stop leaking unrelated thread events into pre-primary startup buffer

[andrei-hasna]

Summary

Fixes startup routing for pre-primary thread notifications (todos task 66ca861a).

During startup the primary thread is configured asynchronously, so there is a window where primary_thread_id is None. Previously handle_server_notification_event / handle_server_request_event routed every thread-targeted event to enqueue_primary_thread_notification/request whenever primary_thread_id was None, and the flush in enqueue_primary_thread_session re-dispatched all buffered events onto the newly-started primary thread. Notifications/requests from unrelated loaded or scheduled threads leaked into the primary buffer and corrupted the primary event stream.

Changes

• Entry point (app_server_events.rs): during the pre-primary window, if the target thread is already tracked (has a channel or is a registered side thread), route directly to that thread instead of buffering it as a pending primary event. Unknown threads still buffer (may be the primary being started).
• Flush (thread_routing.rs): re-derive each buffered event's own thread and route by it — to the started primary when it matches, to an already-tracked thread's own channel otherwise, or drop (with a debug log) when the thread is untracked and unrelated.
• Added ServerNotificationThreadTarget::thread_id() and App::is_tracked_thread / resolve_pending_primary_flush_target helpers.

Tests

• enqueue_primary_thread_session_drops_unrelated_pre_primary_notifications — asserts an unrelated pre-primary notification is dropped rather than replayed onto the primary, while the primary's own buffered event still replays.
• enqueue_primary_thread_session_routes_known_thread_pre_primary_notifications — asserts a known-thread event routes to its own channel, not the primary stream.
• Existing enqueue_primary_thread_session_replays_buffered_approval_after_attach and resolved_buffered_approval_does_not_become_actionable_after_drain still pass (same-thread buffered replay preserved).

Validation: cargo check -p codex-tui, focused cargo test -p codex-tui, cargo fmt, and cargo clippy -p codex-tui --all-targets all clean.

🤖 Generated with Claude Code

[andrei-hasna]

Closing as superseded by #211. Replacement #211 consolidates the current TUI/thread-routing stack and explicitly supersedes this PR. Please continue review on #211.