feat(pipeline): add metrics collection and v2 run mode

- Add RenderStage adapter that handles rendering pipeline
- Add EffectPluginStage with proper EffectContext
- Add DisplayStage with init handling
- Add ItemsStage for pre-fetched items
- Add metrics collection to Pipeline (StageMetrics, FrameMetrics)
- Add get_metrics_summary() and reset_metrics() methods
- Add --pipeline and --pipeline-preset flags for v2 mode
- Add PipelineNode.metrics for self-documenting introspection
- Add introspect_new_pipeline() method with performance data
- Add mise tasks: run-v2, run-v2-demo, run-v2-poetry, run-v2-websocket, run-v2-firehose

engine/config.py

@@ -246,6 +246,10 @@ DEMO = "--demo" in sys.argv
 DEMO_EFFECT_DURATION = 5.0  # seconds per effect
 PIPELINE_DEMO = "--pipeline-demo" in sys.argv
 
+# ─── PIPELINE MODE (new unified architecture) ─────────────
+PIPELINE_MODE = "--pipeline" in sys.argv
+PIPELINE_PRESET = _arg_value("--pipeline-preset", sys.argv) or "demo"
+
 # ─── PRESET MODE ────────────────────────────────────────────
 PRESET = _arg_value("--preset", sys.argv)