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/terminal.py

@@ -0,0 +1,48 @@
+"""
+ANSI terminal display backend.
+"""
+
+import time
+
+
+class TerminalDisplay:
+    """ANSI terminal display backend."""
+
+    width: int = 80
+    height: int = 24
+
+    def __init__(self):
+        self.width = 80
+        self.height = 24
+
+    def init(self, width: int, height: int) -> None:
+        from engine.terminal import CURSOR_OFF
+
+        self.width = width
+        self.height = height
+        print(CURSOR_OFF, end="", flush=True)
+
+    def show(self, buffer: list[str]) -> None:
+        import sys
+
+        t0 = time.perf_counter()
+        sys.stdout.buffer.write("".join(buffer).encode())
+        sys.stdout.flush()
+        elapsed_ms = (time.perf_counter() - t0) * 1000
+
+        from engine.display import get_monitor
+
+        monitor = get_monitor()
+        if monitor:
+            chars_in = sum(len(line) for line in buffer)
+            monitor.record_effect("terminal_display", elapsed_ms, chars_in, chars_in)
+
+    def clear(self) -> None:
+        from engine.terminal import CLR
+
+        print(CLR, end="", flush=True)
+
+    def cleanup(self) -> None:
+        from engine.terminal import CURSOR_ON
+
+        print(CURSOR_ON, end="", flush=True)