feat: Add gallery presets, MultiDisplay support, and viewport tests

- Add ~20 gallery presets covering sources, effects, cameras, displays
- Add MultiDisplay support with --display multi:terminal,pygame syntax
- Fix ViewportFilterStage to recompute layout on viewport_width change
- Add benchmark.py module for hook-based performance testing
- Add viewport resize tests to test_viewport_filter_performance.py

engine/pipeline/adapters.py

@@ -437,8 +437,9 @@ class ViewportFilterStage(Stage):
         viewport_width = ctx.params.viewport_width if ctx.params else 80
         camera_y = ctx.get("camera_y", 0)
 
-        # Recompute layout only when item count changes
-        if len(data) != self._cached_count:
+        # Recompute layout when item count OR viewport width changes
+        cached_width = getattr(self, "_cached_width", None)
+        if len(data) != self._cached_count or cached_width != viewport_width:
             self._layout = []
             y = 0
             from engine.render.blocks import estimate_block_height
@@ -454,6 +455,7 @@ class ViewportFilterStage(Stage):
                 self._layout.append((y, h))
                 y += h
             self._cached_count = len(data)
+            self._cached_width = viewport_width
 
         # Find items visible in [camera_y - buffer, camera_y + viewport_height + buffer]
         buffer_zone = viewport_height