refactor: modularize display backends and add benchmark runner

- Create engine/display/ package with registry pattern
- Move displays to engine/display/backends/ (terminal, null, websocket, sixel)
- Add DisplayRegistry with auto-discovery
- Add benchmark.py for performance testing effects × displays matrix
- Add mise tasks: benchmark, benchmark-json, benchmark-report
- Update controller to use new display module

engine/display/backends/multi.py

@@ -0,0 +1,33 @@
+"""
+Multi display backend - forwards to multiple displays.
+"""
+
+
+class MultiDisplay:
+    """Display that forwards to multiple displays."""
+
+    width: int = 80
+    height: int = 24
+
+    def __init__(self, displays: list):
+        self.displays = displays
+        self.width = 80
+        self.height = 24
+
+    def init(self, width: int, height: int) -> None:
+        self.width = width
+        self.height = height
+        for d in self.displays:
+            d.init(width, height)
+
+    def show(self, buffer: list[str]) -> None:
+        for d in self.displays:
+            d.show(buffer)
+
+    def clear(self) -> None:
+        for d in self.displays:
+            d.clear()
+
+    def cleanup(self) -> None:
+        for d in self.displays:
+            d.cleanup()