feat(gui): warmup status indicator + completion callback (sub-track 4)

Sub-track 4 of startup_speedup_20260606. Adds per-frame GUI feedback
during the AppController's background warmup:

- render_warmup_status_indicator(app): module-level render fn called
  from render_main_interface. Shows 'Warming up... (N/M)' in warning
  color while pending, 'Imports: K failed' in error color on failure,
  or 'All imports ready (M modules)' in success color for 3 seconds
  after completion. Hidden otherwise.
- _on_warmup_complete_callback(app, status): thread-safe callback
  registered with controller.on_warmup_complete() in App._post_init.
  Records timestamp + lock-protected toast list.
- App._post_init: registers the callback.

6 new tests in tests/test_gui_warmup_indicator.py:
- 2 importable-checks (function exists)
- 3 callback-logic tests (timestamp, failures, thread-safety)
- 1 live_gui smoke test (controller exposes warmup_status)

src/gui_2.py

@@ -395,6 +395,15 @@ class App:
 
  def _post_init(self) -> None:
   theme.apply_current()
+  # Register warmup completion callback (sub-track 4 of
+  # startup_speedup_20260606). The callback runs on a background
+  # _io_pool thread; it only sets primitive state on the App, which
+  # is safe. The render_warmup_status_indicator() function reads
+  # the timestamp to show a brief "ready" tag for 3 seconds.
+  if hasattr(self.controller, "on_warmup_complete"):
+   try:
+    self.controller.on_warmup_complete(lambda status: _on_warmup_complete_callback(self, status))
+   except Exception: pass
 
  def _trigger_hot_reload(self) -> bool:
   from src.hot_reloader import HotReloader
@@ -1302,6 +1311,7 @@ if __name__ == "__main__":
 def render_main_interface(app: App) -> None:
  render_error_tint(app)
  render_project_stale_tint(app)
+ render_warmup_status_indicator(app)
  app.perf_monitor.start_frame()
  app._autofocus_response_tab = app.controller._autofocus_response_tab
 
@@ -3781,6 +3791,69 @@ def render_thinking_indicator(app: App) -> None:
   c     = theme.get_color("status_error", alpha=alpha)
   imgui.text_colored(c, "THINKING..."); imgui.same_line()
 
+def _on_warmup_complete_callback(app: App, status: dict) -> None:
+ """
+ Thread-safe callback registered with controller.on_warmup_complete()
+ during App._post_init. Records the completion timestamp; the
+ indicator function uses it to show a brief "ready" tag. Also
+ appends a message to a lock-protected list that the indicator
+ (or the diagnostics panel) can read.
+ Runs on a background _io_pool thread; only sets primitive state.
+ [C: src/gui_2.py:render_warmup_status_indicator, src/gui_2.py:App._post_init, tests/test_gui_warmup_indicator.py:test_callback_sets_timestamp]
+ """
+ try:
+  app._warmup_completion_ts = time.time()
+  pending = status.get("pending", [])
+  completed = status.get("completed", [])
+  failed = status.get("failed", [])
+  total = len(pending) + len(completed) + len(failed)
+  if failed: msg = f"Warmup finished with {len(failed)} failures ({total} modules)"
+  else:      msg = f"All imports ready ({total} modules)"
+  if not hasattr(app, "_warmup_toast_lock"):
+   import threading as _threading
+   app._warmup_toast_lock = _threading.Lock()
+  with app._warmup_toast_lock:
+   if not hasattr(app, "_warmup_toast_messages"): app._warmup_toast_messages = []
+   app._warmup_toast_messages.append((time.time(), msg))
+ except Exception: pass
+
+def render_warmup_status_indicator(app: App) -> None:
+ """
+ Renders a per-frame warmup status line. Shows the progress of
+ AppController's background warmup (Phase 2 of
+ startup_speedup_20260606). Hidden when the controller has no warmup
+ or warmup is done with no failures. Shows a transient "ready" tag
+ for 3 seconds after completion.
+ [C: src/gui_2.py:App._post_init, src/gui_2.py:render_main_interface, tests/test_gui_warmup_indicator.py:test_render_warmup_indicator_function_exists, tests/test_gui_warmup_indicator.py:test_callback_sets_timestamp]
+ """
+ controller = getattr(app, "controller", None)
+ if controller is None: return
+ if not hasattr(controller, "warmup_status"): return
+ try:
+  status = controller.warmup_status()
+ except Exception: return
+ pending = status.get("pending", [])
+ completed = status.get("completed", [])
+ failed = status.get("failed", [])
+ if pending:
+  total = len(pending) + len(completed) + len(failed)
+  done = len(completed) + len(failed)
+  c = theme.get_color("status_warning")
+  imgui.text_colored(c, f"Warming up... ({done}/{total})")
+  return
+ if failed:
+  c = theme.get_color("status_error")
+  imgui.text_colored(c, f"Imports: {len(failed)} failed")
+  return
+ # Steady state + transient 3s "ready" tag after completion.
+ ts = getattr(app, "_warmup_completion_ts", 0.0)
+ if ts > 0 and (time.time() - ts) < 3.0:
+  total = len(completed) + len(failed)
+  c = theme.get_color("status_success")
+  imgui.text_colored(c, f"All imports ready ({total} modules)")
+  return
+ # No render: warmup done, no failures, transient window expired.
+
 def render_synthesis_panel(app: App) -> None:
  """