feat: add partial update support with caller-declared dirty tracking

- Add PartialUpdate dataclass and supports_partial_updates to EffectPlugin
- Add dirty region tracking to Canvas (mark_dirty, get_dirty_rows, etc.)
- Canvas auto-marks dirty on put_region, put_text, fill
- CanvasStage exposes dirty rows via pipeline context
- EffectChain creates PartialUpdate and calls process_partial() for optimized effects
- HudEffect implements process_partial() to skip processing when rows 0-2 not dirty
- This enables effects to skip work when canvas regions haven't changed

engine/pipeline/adapters.py

@@ -530,6 +530,14 @@ class CanvasStage(Stage):
 
             self._canvas = Canvas(width=self._width, height=self._height)
             ctx.set("canvas", self._canvas)
+
+        # Get dirty regions from canvas and expose via context
+        # Effects can access via ctx.get_state("canvas.dirty_rows")
+        if self._canvas.is_dirty():
+            dirty_rows = self._canvas.get_dirty_rows()
+            ctx.set_state("canvas.dirty_rows", dirty_rows)
+            ctx.set_state("canvas.dirty_regions", self._canvas.get_dirty_regions())
+
         return data
 
     def get_canvas(self):