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/display/__init__.py

@@ -147,6 +147,31 @@ class DisplayRegistry:
 
         cls._initialized = True
 
+    @classmethod
+    def create_multi(cls, names: list[str]) -> "Display | None":
+        """Create a MultiDisplay from a list of backend names.
+
+        Args:
+            names: List of display backend names (e.g., ["terminal", "pygame"])
+
+        Returns:
+            MultiDisplay instance or None if any backend fails
+        """
+        from engine.display.backends.multi import MultiDisplay
+
+        displays = []
+        for name in names:
+            backend = cls.create(name)
+            if backend:
+                displays.append(backend)
+            else:
+                return None
+
+        if not displays:
+            return None
+
+        return MultiDisplay(displays)
+
 
 def get_monitor():
     """Get the performance monitor."""