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

engine/pipeline/registry.py

@@ -89,17 +89,27 @@ def discover_stages() -> None:
     try:
         from engine.sources_v2 import (
             HeadlinesDataSource,
-            PipelineDataSource,
             PoetryDataSource,
         )
 
         StageRegistry.register("source", HeadlinesDataSource)
         StageRegistry.register("source", PoetryDataSource)
-        StageRegistry.register("source", PipelineDataSource)
 
         StageRegistry._categories["source"]["headlines"] = HeadlinesDataSource
         StageRegistry._categories["source"]["poetry"] = PoetryDataSource
-        StageRegistry._categories["source"]["pipeline"] = PipelineDataSource
+    except ImportError:
+        pass
+
+    # Register pipeline introspection source
+    try:
+        from engine.pipeline_sources.pipeline_introspection import (
+            PipelineIntrospectionSource,
+        )
+
+        StageRegistry.register("source", PipelineIntrospectionSource)
+        StageRegistry._categories["source"]["pipeline-inspect"] = (
+            PipelineIntrospectionSource
+        )
     except ImportError:
         pass