refactor: remove legacy demo code, integrate metrics via pipeline context

- Remove ~700 lines of legacy code from app.py (run_demo_mode, run_pipeline_demo,
  run_preset_mode, font picker, effects picker)
- HUD now reads metrics from pipeline context (first-class citizen) with fallback
  to global monitor for backwards compatibility
- Add validate_signal_flow() for PureData-style type validation in presets
- Update MicSensor documentation (self-contained, doesn't use MicMonitor)
- Delete test_app.py (was testing removed legacy code)
- Update AGENTS.md with pipeline architecture documentation

engine/mic.py

@@ -4,7 +4,7 @@ Gracefully degrades if sounddevice/numpy are unavailable.
 
 .. deprecated::
     For pipeline integration, use :class:`engine.sensors.mic.MicSensor` instead.
-    MicMonitor is still used as the backend for MicSensor.
+    MicSensor is a self-contained implementation and does not use MicMonitor.
 """
 
 import atexit