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

tests/test_pipeline.py

@@ -31,7 +31,7 @@ class TestStageRegistry:
         sources = StageRegistry.list("source")
         assert "HeadlinesDataSource" in sources
         assert "PoetryDataSource" in sources
-        assert "PipelineDataSource" in sources
+        assert "PipelineIntrospectionSource" in sources
 
     def test_discover_stages_registers_displays(self):
         """discover_stages registers display stages."""