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)
