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)

engine/display/backends/pygame.py

@@ -41,6 +41,7 @@ class PygameDisplay:
         self._quit_requested = False
         self._last_frame_time = 0.0
         self._frame_period = 1.0 / target_fps if target_fps > 0 else 0
+        self._glyph_cache = {}
 
     def _get_font_path(self) -> str | None:
         """Get font path for rendering."""
@@ -191,6 +192,8 @@ class PygameDisplay:
 
         self._screen.fill((0, 0, 0))
 
+        blit_list = []
+
         for row_idx, line in enumerate(buffer[: self.height]):
             if row_idx >= self.height:
                 break
@@ -202,15 +205,24 @@ class PygameDisplay:
                 if not text:
                     continue
 
-                if bg != (0, 0, 0):
-                    bg_surface = self._font.render(text, True, fg, bg)
-                    self._screen.blit(bg_surface, (x_pos, row_idx * self.cell_height))
-                else:
-                    text_surface = self._font.render(text, True, fg)
-                    self._screen.blit(text_surface, (x_pos, row_idx * self.cell_height))
+                # Use None as key for no background
+                bg_key = bg if bg != (0, 0, 0) else None
+                cache_key = (text, fg, bg_key)
 
+                if cache_key not in self._glyph_cache:
+                    # Render and cache
+                    if bg_key is not None:
+                        self._glyph_cache[cache_key] = self._font.render(
+                            text, True, fg, bg_key
+                        )
+                    else:
+                        self._glyph_cache[cache_key] = self._font.render(text, True, fg)
+
+                surface = self._glyph_cache[cache_key]
+                blit_list.append((surface, (x_pos, row_idx * self.cell_height)))
                 x_pos += self._font.size(text)[0]
 
+        self._screen.blits(blit_list)
         self._pygame.display.flip()
 
         elapsed_ms = (time.perf_counter() - t0) * 1000