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

effects_plugins/hud.py

@@ -1,4 +1,3 @@
-from engine.effects.performance import get_monitor
 from engine.effects.types import EffectConfig, EffectContext, EffectPlugin
 
 
@@ -8,15 +7,34 @@ class HudEffect(EffectPlugin):
 
     def process(self, buf: list[str], ctx: EffectContext) -> list[str]:
         result = list(buf)
-        monitor = get_monitor()
+
+        # Read metrics from pipeline context (first-class citizen)
+        # Falls back to global monitor for backwards compatibility
+        metrics = ctx.get_state("metrics")
+        if not metrics:
+            # Fallback to global monitor for backwards compatibility
+            from engine.effects.performance import get_monitor
+
+            monitor = get_monitor()
+            if monitor:
+                stats = monitor.get_stats()
+                if stats and "pipeline" in stats:
+                    metrics = stats
 
         fps = 0.0
         frame_time = 0.0
-        if monitor:
-            stats = monitor.get_stats()
-            if stats and "pipeline" in stats:
-                frame_time = stats["pipeline"].get("avg_ms", 0.0)
-                frame_count = stats.get("frame_count", 0)
+        if metrics:
+            if "error" in metrics:
+                pass  # No metrics available yet
+            elif "pipeline" in metrics:
+                frame_time = metrics["pipeline"].get("avg_ms", 0.0)
+                frame_count = metrics.get("frame_count", 0)
+                if frame_count > 0 and frame_time > 0:
+                    fps = 1000.0 / frame_time
+            elif "avg_ms" in metrics:
+                # Direct metrics format
+                frame_time = metrics.get("avg_ms", 0.0)
+                frame_count = metrics.get("frame_count", 0)
                 if frame_count > 0 and frame_time > 0:
                     fps = 1000.0 / frame_time
 
@@ -44,11 +62,17 @@ class HudEffect(EffectPlugin):
             f"\033[2;1H\033[38;5;45mEFFECT:\033[0m \033[1;38;5;227m{effect_name:12s}\033[0m \033[38;5;245m|\033[0m {bar} \033[38;5;245m|\033[0m \033[38;5;219m{effect_intensity * 100:.0f}%\033[0m"
         )
 
-        from engine.effects import get_effect_chain
+        # Try to get pipeline order from context
+        pipeline_order = ctx.get_state("pipeline_order")
+        if pipeline_order:
+            pipeline_str = ",".join(pipeline_order)
+        else:
+            # Fallback to legacy effect chain
+            from engine.effects import get_effect_chain
 
-        chain = get_effect_chain()
-        order = chain.get_order()
-        pipeline_str = ",".join(order) if order else "(none)"
+            chain = get_effect_chain()
+            order = chain.get_order() if chain else []
+            pipeline_str = ",".join(order) if order else "(none)"
         hud_lines.append(f"\033[3;1H\033[38;5;44mPIPELINE:\033[0m {pipeline_str}")
 
         for i, line in enumerate(hud_lines):