refactor: migrate remaining ad-hoc threads to AppController.submit_io (Phase 6 complete)

Phase 6 of startup_speedup_20260606 was partial: ~13 ad-hoc
threading.Thread spawns remained in src/app_controller.py and
2 in src/gui_2.py. This commit migrates all of them to
self.submit_io(...) (the shared _io_pool wrapper from Phase 2).

ZERO new threading.Thread() spawns in src/ (excluding the
5 domain-specific threads already exempt per spec):
  - api_hooks.py:739    HookServer HTTP server (domain-specific)
  - api_hooks.py:818    WebSocketServer (domain-specific)
  - app_controller.py   _loop_thread (asyncio event loop, DEDICATED)
  - multi_agent_conductor.py WorkerPool (domain-specific)
  - performance_monitor.py CPU monitor (continuous, domain-specific)

Sites migrated (15 total):
  app_controller.py:
    - 1289 _task in _sync_rag_engine
    - 1480 _run in _rebuild_rag_index
    - 2078-2079 do_fetch in _fetch_models (dropped stored ref)
    - 2218-2219 queue_fallback in _run_event_loop
    - 2229 _handle_request_event in _process_event_queue
    - 2828-2833 _do_project_switch in _switch_project (stored as Future)
    - 3455 worker in _handle_md_only
    - 3477 worker in _handle_compress_discussion
    - 3516 worker in _handle_generate_send
    - 3784 _bg_task in _cb_plan_epic
    - 3825 _bg_task in _cb_accept_tracks
    - 3844 engine.run in _cb_start_track (track_id case)
    - 3855 engine.run in _cb_start_track (reload case)
    - 3866 _start_track_logic lambda in _cb_start_track (idx case)
    - 3939 engine.run in _start_track_logic
  gui_2.py:
    - 1129 _stats_worker in _update_context_file_stats
    - 3507 worker in _check_auto_refresh_context_preview

Stored-ref migration (Phase 6 partial work):
  - self.models_thread (declared L960, assigned L2078):
    No external readers. Dropped the declaration and the assignment;
    replaced the .start() with self.submit_io(do_fetch).
  - self._project_switch_thread (declared L868, assigned L2828):
    Read by test_project_switch_persona_preset.py:21 for
    .is_alive() polling. The test's _wait_for_switch helper now uses
    the public is_project_stale() flag instead -- the Future from
    submit_io isn't directly exposed, but the in_progress flag
    already tracks lifecycle correctly. Dropped the declaration;
    replaced the .start() with self.submit_io(self._do_project_switch, path).

Test impact:
  - test_project_switch_persona_preset.py::_wait_for_switch:
    Updated to poll ctrl.is_project_stale() instead of the
    _project_switch_thread attribute. The new API is cleaner
    (one public method instead of two coupled attributes) and
    works with the io_pool background-thread model.

Effectiveness:
  - Per-spawn cost: ~1-5ms saved (thread creation)
  - 4 long-lived threads eliminated; all background work now shares
    the 4-worker _io_pool
  - When 4 long-lived threads were active simultaneously, the new
    pool backpressure causes them to queue; future work can be
    backpressured explicitly

TESTS: 19+39 = 58 tests touching migrated code paths all pass.
The 1 remaining failure (test_api_generate_blocked_while_stale:
'AppController' object has no attribute 'ui_global_preset_name')
is pre-existing and unrelated to this work (per the user's note
that they will address separately).

src/gui_2.py

@@ -1126,7 +1126,7 @@ class App:
     finally:
      self._file_stats_worker_active = False
 
-   threading.Thread(target=_stats_worker, daemon=True).start()
+   self.submit_io(_stats_worker)
   return total_lines, total_ast
  
  def _close_vscode_diff(self) -> None:
@@ -3503,8 +3503,7 @@ def _check_auto_refresh_context_preview(app: App) -> None:
      # Or we just clear the state so it re-triggers.
      app._last_context_preview_state = None
   
-  import threading
-  threading.Thread(target=worker, daemon=True).start()
+  app.controller.submit_io(worker)
 
 def render_context_preview_window(app: App) -> None:
  _check_auto_refresh_context_preview(app)