feat(startup): first-frame detection + startup_timeline API

Adds per-AppController startup timing instrumentation to answer
'did the warmup block the first frame?'

AppController.__init__ records _init_start_ts at entry (cold-start anchor).
WarmupManager.on_complete callback stamps _warmup_done_ts.
App.render_main_interface (gui_2.py) calls mark_first_frame_rendered()
on its first call, which stamps _first_frame_ts and logs the timeline.

New public API on AppController:
- init_start_ts (property): float
- warmup_done_ts (property): Optional[float]
- first_frame_ts (property): Optional[float]
- mark_first_frame_rendered(ts=None): idempotent; logs to stderr
- startup_timeline() -> dict with all timestamps + precomputed deltas:
  warmup_ms, first_frame_after_init_ms, first_frame_after_warmup_ms

Stderr log on warmup done:
  [startup] warmup done in 1186.2ms (first frame rendered Nms BEFORE/AFTER)

Stderr log on first frame:
  [startup] first frame at Xms after init (warmup took Yms) (rendered Zms BEFORE/AFTER warmup done)

Hook API:
- GET /api/startup_timeline
- ApiHookClient.get_startup_timeline() -> dict

5 new tests in test_warmup_canaries.py covering all the new methods.
All 18 canary tests + 10 api_hooks tests + 6 gui_indicator tests pass.

Script scripts/apply_startup_timeline.py is included as a reference
for the multi-edit pattern (the proper MCP-equivalent tools will be
added later per the edit_workflow doc).

src/api_hook_client.py

@@ -329,6 +329,16 @@ class ApiHookClient:
   result = self._make_request('GET', '/api/warmup_canaries') or {}
   return result.get("canaries", []) if isinstance(result, dict) else []
 
+ def get_startup_timeline(self) -> dict[str, Any]:
+  """
+  Returns the startup timeline: dict with init_start_ts, warmup_done_ts,
+  first_frame_ts, warmup_ms, first_frame_after_init_ms,
+  first_frame_after_warmup_ms. Lets external clients answer
+  'did the warmup block the first frame?'.
+  [C: tests/test_api_hooks_warmup.py:test_live_startup_timeline_endpoint]
+  """
+  return self._make_request('GET', '/api/startup_timeline') or {}
+
 #endregion: Diagnostics
 
 #region: Project