feat(display): add configurable multi-backend display system

engine/display.py

@@ -100,3 +100,30 @@ class NullDisplay:
 
     def cleanup(self) -> None:
         pass
+
+
+class MultiDisplay:
+    """Display that forwards to multiple displays."""
+
+    def __init__(self, displays: list[Display]):
+        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()