feat(pygame): add glyph caching for performance improvement

- Add _glyph_cache dict to PygameDisplay.__init__
- Cache font.render() results per (char, fg, bg) combination
- Use blits() for batch rendering instead of individual blit calls
- Add TestRenderBorder tests (8 new tests) for border rendering
- Update NullDisplay.show() to support border=True for consistency
- Add test_show_with_border_uses_render_border for TerminalDisplay

Closes #28

engine/display/backends/null.py

@@ -33,10 +33,25 @@ class NullDisplay:
         self._last_buffer = None
 
     def show(self, buffer: list[str], border: bool = False) -> None:
-        from engine.display import get_monitor
+        from engine.display import get_monitor, render_border
+
+        # Get FPS for border (if available)
+        fps = 0.0
+        frame_time = 0.0
+        monitor = get_monitor()
+        if monitor:
+            stats = monitor.get_stats()
+            avg_ms = stats.get("pipeline", {}).get("avg_ms", 0) if stats else 0
+            frame_count = stats.get("frame_count", 0) if stats else 0
+            if avg_ms and frame_count > 0:
+                fps = 1000.0 / avg_ms
+                frame_time = avg_ms
+
+        # Apply border if requested (same as terminal display)
+        if border:
+            buffer = render_border(buffer, self.width, self.height, fps, frame_time)
 
         self._last_buffer = buffer
-        monitor = get_monitor()
         if monitor:
             t0 = time.perf_counter()
             chars_in = sum(len(line) for line in buffer)