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/effects/chain.py

@@ -2,7 +2,7 @@ import time
 
 from engine.effects.performance import PerformanceMonitor, get_monitor
 from engine.effects.registry import EffectRegistry
-from engine.effects.types import EffectContext
+from engine.effects.types import EffectContext, PartialUpdate
 
 
 class EffectChain:
@@ -51,6 +51,18 @@ class EffectChain:
         frame_number = ctx.frame_number
         monitor.start_frame(frame_number)
 
+        # Get dirty regions from canvas via context (set by CanvasStage)
+        dirty_rows = ctx.get_state("canvas.dirty_rows")
+
+        # Create PartialUpdate for effects that support it
+        full_buffer = dirty_rows is None or len(dirty_rows) == 0
+        partial = PartialUpdate(
+            rows=None,
+            cols=None,
+            dirty=dirty_rows,
+            full_buffer=full_buffer,
+        )
+
         frame_start = time.perf_counter()
         result = list(buf)
         for name in self._order:
@@ -59,7 +71,11 @@ class EffectChain:
                 chars_in = sum(len(line) for line in result)
                 effect_start = time.perf_counter()
                 try:
-                    result = plugin.process(result, ctx)
+                    # Use process_partial if supported, otherwise fall back to process
+                    if getattr(plugin, "supports_partial_updates", False):
+                        result = plugin.process_partial(result, ctx, partial)
+                    else:
+                        result = plugin.process(result, ctx)
                 except Exception:
                     plugin.config.enabled = False
                 elapsed = time.perf_counter() - effect_start