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

effects_plugins/hud.py

@@ -1,9 +1,35 @@
-from engine.effects.types import EffectConfig, EffectContext, EffectPlugin
+from engine.effects.types import (
+    EffectConfig,
+    EffectContext,
+    EffectPlugin,
+    PartialUpdate,
+)
 
 
 class HudEffect(EffectPlugin):
     name = "hud"
     config = EffectConfig(enabled=True, intensity=1.0)
+    supports_partial_updates = True  # Enable partial update optimization
+
+    # Cache last HUD content to detect changes
+    _last_hud_content: tuple | None = None
+
+    def process_partial(
+        self, buf: list[str], ctx: EffectContext, partial: PartialUpdate
+    ) -> list[str]:
+        # If full buffer requested, process normally
+        if partial.full_buffer:
+            return self.process(buf, ctx)
+
+        # If HUD rows (0, 1, 2) aren't dirty, skip processing
+        if partial.dirty:
+            hud_rows = {0, 1, 2}
+            dirty_hud_rows = partial.dirty & hud_rows
+            if not dirty_hud_rows:
+                return buf  # Nothing for HUD to do
+
+        # Proceed with full processing
+        return self.process(buf, ctx)
 
     def process(self, buf: list[str], ctx: EffectContext) -> list[str]:
         result = list(buf)