feat: add pipeline introspection demo mode

- Add PipelineIntrospectionSource that renders live ASCII DAG with metrics
- Add PipelineMetricsSensor exposing pipeline performance as sensor values
- Add PipelineIntrospectionDemo controller with 3-phase animation:
  - Phase 1: Toggle effects one at a time (3s each)
  - Phase 2: LFO drives intensity default→max→min→default
  - Phase 3: All effects with shared LFO (infinite loop)
- Add pipeline-inspect preset
- Add get_frame_times() to Pipeline for sparkline data
- Add tests for new components
- Update mise.toml with pipeline-inspect preset task

AGENTS.md

@@ -161,7 +161,7 @@ The project uses pytest with strict marker enforcement. Test configuration is in
 
 ### Test Coverage Strategy
 
-Current coverage: 56% (433 tests)
+Current coverage: 56% (434 tests)
 
 Key areas with lower coverage (acceptable for now):
 - **app.py** (8%): Main entry point - integration heavy, requires terminal
@@ -186,11 +186,18 @@ Performance regression tests are in `tests/test_benchmark.py` with `@pytest.mark
 
 ## Architecture Notes
 
-- **ntfy.py** and **mic.py** are standalone modules with zero internal dependencies
+- **ntfy.py** - standalone notification poller with zero internal dependencies
+- **sensors/** - Sensor framework (MicSensor, OscillatorSensor) for real-time input
 - **eventbus.py** provides thread-safe event publishing for decoupled communication
-- **controller.py** coordinates ntfy/mic monitoring and event publishing
 - **effects/** - plugin architecture with performance monitoring
-- The render pipeline: fetch → render → effects → scroll → terminal output
+- The new pipeline architecture: source → render → effects → display
+
+#### Canvas & Camera
+
+- **Canvas** (`engine/canvas.py`): 2D rendering surface with dirty region tracking
+- **Camera** (`engine/camera.py`): Viewport controller for scrolling content
+
+The Canvas tracks dirty regions automatically when content is written (via `put_region`, `put_text`, `fill`), enabling partial buffer updates for optimized effect processing.
 
 ### Pipeline Architecture
 
@@ -214,9 +221,24 @@ Stages declare capabilities (what they provide) and dependencies (what they need
 - **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
+- **PipelineMetricsSensor** (`engine/sensors/pipeline_metrics.py`): Exposes pipeline metrics as sensor values
 
 Sensors support param bindings to drive effect parameters in real-time.
 
+#### Pipeline Introspection
+
+- **PipelineIntrospectionSource** (`engine/pipeline_sources/pipeline_introspection.py`): Renders live ASCII visualization of pipeline DAG with metrics
+- **PipelineIntrospectionDemo** (`engine/pipeline/pipeline_introspection_demo.py`): 3-phase demo controller for effect animation
+
+Preset: `pipeline-inspect` - Live pipeline introspection with DAG and performance metrics
+
+#### Partial Update Support
+
+Effect plugins can opt-in to partial buffer updates for performance optimization:
+- Set `supports_partial_updates = True` on the effect class
+- Implement `process_partial(buf, ctx, partial)` method
+- The `PartialUpdate` dataclass indicates which regions changed
+
 ### Preset System
 
 Presets use TOML format (no external dependencies):