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

AGENTS.md

@@ -161,7 +161,7 @@ The project uses pytest with strict marker enforcement. Test configuration is in
 
 ### Test Coverage Strategy
 
-Current coverage: 56% (336 tests)
+Current coverage: 56% (433 tests)
 
 Key areas with lower coverage (acceptable for now):
 - **app.py** (8%): Main entry point - integration heavy, requires terminal
@@ -192,6 +192,47 @@ Performance regression tests are in `tests/test_benchmark.py` with `@pytest.mark
 - **effects/** - plugin architecture with performance monitoring
 - The render pipeline: fetch → render → effects → scroll → terminal output
 
+### Pipeline Architecture
+
+The new Stage-based pipeline architecture provides capability-based dependency resolution:
+
+- **Stage** (`engine/pipeline/core.py`): Base class for pipeline stages
+- **Pipeline** (`engine/pipeline/controller.py`): Executes stages with capability-based dependency resolution
+- **StageRegistry** (`engine/pipeline/registry.py`): Discovers and registers stages
+- **Stage Adapters** (`engine/pipeline/adapters.py`): Wraps existing components as stages
+
+#### Capability-Based Dependencies
+
+Stages declare capabilities (what they provide) and dependencies (what they need). The Pipeline resolves dependencies using prefix matching:
+- `"source"` matches `"source.headlines"`, `"source.poetry"`, etc.
+- This allows flexible composition without hardcoding specific stage names
+
+#### Sensor Framework
+
+- **Sensor** (`engine/sensors/__init__.py`): Base class for real-time input sensors
+- **SensorRegistry**: Discovers available sensors
+- **SensorStage**: Pipeline adapter that provides sensor values to effects
+- **MicSensor** (`engine/sensors/mic.py`): Self-contained microphone input
+- **OscillatorSensor** (`engine/sensors/oscillator.py`): Test sensor for development
+
+Sensors support param bindings to drive effect parameters in real-time.
+
+### Preset System
+
+Presets use TOML format (no external dependencies):
+
+- Built-in: `engine/presets.toml`
+- User config: `~/.config/mainline/presets.toml`
+- Local override: `./presets.toml`
+
+- **Preset loader** (`engine/pipeline/preset_loader.py`): Loads and validates presets
+- **PipelinePreset** (`engine/pipeline/presets.py`): Dataclass for preset configuration
+
+Functions:
+- `validate_preset()` - Validate preset structure
+- `validate_signal_path()` - Detect circular dependencies
+- `generate_preset_toml()` - Generate skeleton preset
+
 ### Display System
 
 - **Display abstraction** (`engine/display/`): swap display backends via the Display protocol

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):

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

engine/pipeline/preset_loader.py

@@ -152,6 +152,64 @@ def validate_preset(preset: dict[str, Any]) -> list[str]:
     return errors
 
 
+def validate_signal_flow(stages: list[dict]) -> list[str]:
+    """Validate signal flow based on inlet/outlet types.
+
+    This validates that the preset's stage configuration produces valid
+    data flow using the PureData-style type system.
+
+    Args:
+        stages: List of stage configs with 'name', 'category', 'inlet_types', 'outlet_types'
+
+    Returns:
+        List of errors (empty if valid)
+    """
+    errors: list[str] = []
+
+    if not stages:
+        errors.append("Signal flow is empty")
+        return errors
+
+    # Define expected types for each category
+    type_map = {
+        "source": {"inlet": "NONE", "outlet": "SOURCE_ITEMS"},
+        "data": {"inlet": "ANY", "outlet": "SOURCE_ITEMS"},
+        "transform": {"inlet": "SOURCE_ITEMS", "outlet": "TEXT_BUFFER"},
+        "effect": {"inlet": "TEXT_BUFFER", "outlet": "TEXT_BUFFER"},
+        "overlay": {"inlet": "TEXT_BUFFER", "outlet": "TEXT_BUFFER"},
+        "camera": {"inlet": "TEXT_BUFFER", "outlet": "TEXT_BUFFER"},
+        "display": {"inlet": "TEXT_BUFFER", "outlet": "NONE"},
+        "render": {"inlet": "SOURCE_ITEMS", "outlet": "TEXT_BUFFER"},
+    }
+
+    # Check stage order and type compatibility
+    for i, stage in enumerate(stages):
+        category = stage.get("category", "unknown")
+        name = stage.get("name", f"stage_{i}")
+
+        if category not in type_map:
+            continue  # Skip unknown categories
+
+        expected = type_map[category]
+
+        # Check against previous stage
+        if i > 0:
+            prev = stages[i - 1]
+            prev_category = prev.get("category", "unknown")
+            if prev_category in type_map:
+                prev_outlet = type_map[prev_category]["outlet"]
+                inlet = expected["inlet"]
+
+                # Validate type compatibility
+                if inlet != "ANY" and prev_outlet != "ANY" and inlet != prev_outlet:
+                    errors.append(
+                        f"Type mismatch at '{name}': "
+                        f"expects {inlet} but previous stage outputs {prev_outlet}"
+                    )
+
+    return errors
+
+
 def validate_signal_path(stages: list[str]) -> list[str]:
     """Validate signal path for circular dependencies and connectivity.
 

tests/test_app.py

@@ -1,55 +0,0 @@
-"""
-Tests for engine.app module.
-"""
-
-from engine.app import _normalize_preview_rows
-
-
-class TestNormalizePreviewRows:
-    """Tests for _normalize_preview_rows function."""
-
-    def test_empty_rows(self):
-        """Empty input returns empty list."""
-        result = _normalize_preview_rows([])
-        assert result == [""]
-
-    def test_strips_left_padding(self):
-        """Left padding is stripped."""
-        result = _normalize_preview_rows(["  content", "  more"])
-        assert all(not r.startswith(" ") for r in result)
-
-    def test_preserves_content(self):
-        """Content is preserved."""
-        result = _normalize_preview_rows(["  hello world  "])
-        assert "hello world" in result[0]
-
-    def test_handles_all_empty_rows(self):
-        """All empty rows returns single empty string."""
-        result = _normalize_preview_rows(["", "   ", ""])
-        assert result == [""]
-
-
-class TestAppConstants:
-    """Tests for app module constants."""
-
-    def test_title_defined(self):
-        """TITLE is defined."""
-        from engine.app import TITLE
-
-        assert len(TITLE) > 0
-
-    def test_title_lines_are_strings(self):
-        """TITLE contains string lines."""
-        from engine.app import TITLE
-
-        assert all(isinstance(line, str) for line in TITLE)
-
-
-class TestAppImports:
-    """Tests for app module imports."""
-
-    def test_app_imports_without_error(self):
-        """Module imports without error."""
-        from engine import app
-
-        assert app is not None