feat(api_hooks): add /api/warmup_status and /api/warmup_wait endpoints (sub-track 3)

Sub-track 3 of startup_speedup_20260606. Builds on the Phase 7 minimal
work at b464d1fe which only added warmup_status to /api/gui/diagnostics.

New dedicated endpoints:
- GET /api/warmup_status -> controller.warmup_status() (cheap, lock-guarded)
- GET /api/warmup_wait?timeout=N -> controller.wait_for_warmup(timeout)
  then returns the final status. Default 30s.

Both callable from external clients via ApiHookClient.get_warmup_status()
and ApiHookClient.get_warmup_wait(timeout=30.0).

7 new tests in tests/test_api_hooks_warmup.py (5 unit + 2 live_gui).
All 7 pass.

src/api_hook_client.py

@@ -301,6 +301,23 @@ class ApiHookClient:
   """
   return self._make_request('GET', '/api/performance') or {}
 
+ def get_warmup_status(self) -> dict[str, Any]:
+  """
+  Returns the current warmup status: {pending, completed, failed}.
+  [C: tests/test_api_hooks_warmup.py:test_get_warmup_status_calls_correct_endpoint, tests/test_api_hooks_warmup.py:test_get_warmup_status_handles_empty_response, tests/test_api_hooks_warmup.py:test_live_warmup_status_endpoint]
+  """
+  return self._make_request('GET', '/api/warmup_status') or {}
+
+ def get_warmup_wait(self, timeout: float = 30.0) -> dict[str, Any]:
+  """
+  Blocks server-side up to `timeout` seconds waiting for the warmup to
+  complete, then returns the final status. Useful for external clients
+  that need to wait until the system is fully ready before issuing AI
+  requests.
+  [C: tests/test_api_hooks_warmup.py:test_get_warmup_wait_passes_timeout_as_query_string, tests/test_api_hooks_warmup.py:test_get_warmup_wait_uses_default_timeout_when_unspecified, tests/test_api_hooks_warmup.py:test_get_warmup_wait_handles_empty_response, tests/test_api_hooks_warmup.py:test_live_warmup_wait_endpoint_completes]
+  """
+  return self._make_request('GET', f'/api/warmup_wait?timeout={timeout}') or {}
+
 #endregion: Diagnostics
 
 #region: Project