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

tests/test_display.py

@@ -3,9 +3,11 @@ Tests for engine.display module.
 """
 
 import sys
-from unittest.mock import MagicMock
+from unittest.mock import MagicMock, patch
 
-from engine.display import DisplayRegistry, NullDisplay, TerminalDisplay
+import pytest
+
+from engine.display import DisplayRegistry, NullDisplay, TerminalDisplay, render_border
 from engine.display.backends.multi import MultiDisplay
 
 
@@ -133,7 +135,6 @@ class TestTerminalDisplay:
         The fix adds \\033[H\\033[J (cursor home + erase down) before each frame.
         """
         from io import BytesIO
-        from unittest.mock import patch
 
         display = TerminalDisplay()
         display.init(80, 24)
@@ -157,7 +158,6 @@ class TestTerminalDisplay:
         Regression test: Ensures each show() call includes the clear sequence.
         """
         from io import BytesIO
-        from unittest.mock import patch
 
         # Use target_fps=0 to disable frame skipping in test
         display = TerminalDisplay(target_fps=0)
@@ -193,6 +193,40 @@ class TestTerminalDisplay:
         # returns different values each call
         assert len(set(dims)) == 1, f"Dimensions should be stable, got: {set(dims)}"
 
+    def test_show_with_border_uses_render_border(self):
+        """show with border=True calls render_border with FPS."""
+        from unittest.mock import MagicMock
+
+        display = TerminalDisplay()
+        display.init(80, 24)
+
+        buffer = ["line1", "line2"]
+
+        # Mock get_monitor to provide FPS
+        mock_monitor = MagicMock()
+        mock_monitor.get_stats.return_value = {
+            "pipeline": {"avg_ms": 16.5},
+            "frame_count": 100,
+        }
+
+        # Mock render_border to verify it's called
+        with (
+            patch("engine.display.get_monitor", return_value=mock_monitor),
+            patch("engine.display.render_border", wraps=render_border) as mock_render,
+        ):
+            display.show(buffer, border=True)
+
+            # Verify render_border was called with correct arguments
+            assert mock_render.called
+            args, kwargs = mock_render.call_args
+            # Arguments: buffer, width, height, fps, frame_time (positional)
+            assert args[0] == buffer
+            assert args[1] == 80
+            assert args[2] == 24
+            assert args[3] == pytest.approx(60.6, rel=0.1)  # fps = 1000/16.5
+            assert args[4] == pytest.approx(16.5, rel=0.1)
+            assert kwargs == {}  # no keyword arguments
+
 
 class TestNullDisplay:
     """Tests for NullDisplay class."""
@@ -241,6 +275,92 @@ class TestNullDisplay:
         assert display._last_buffer == ["second"]
 
 
+class TestRenderBorder:
+    """Tests for render_border function."""
+
+    def test_render_border_adds_corners(self):
+        """render_border adds corner characters."""
+        from engine.display import render_border
+
+        buffer = ["hello", "world"]
+        result = render_border(buffer, width=10, height=5)
+
+        assert result[0][0] in "┌┎┍"  # top-left
+        assert result[0][-1] in "┐┒┓"  # top-right
+        assert result[-1][0] in "└┚┖"  # bottom-left
+        assert result[-1][-1] in "┘┛┙"  # bottom-right
+
+    def test_render_border_dimensions(self):
+        """render_border output matches requested dimensions."""
+        from engine.display import render_border
+
+        buffer = ["line1", "line2", "line3"]
+        result = render_border(buffer, width=20, height=10)
+
+        # Output should be exactly height lines
+        assert len(result) == 10
+        # Each line should be exactly width characters
+        for line in result:
+            assert len(line) == 20
+
+    def test_render_border_with_fps(self):
+        """render_border includes FPS in top border when provided."""
+        from engine.display import render_border
+
+        buffer = ["test"]
+        result = render_border(buffer, width=20, height=5, fps=60.0)
+
+        top_line = result[0]
+        assert "FPS:60" in top_line or "FPS: 60" in top_line
+
+    def test_render_border_with_frame_time(self):
+        """render_border includes frame time in bottom border when provided."""
+        from engine.display import render_border
+
+        buffer = ["test"]
+        result = render_border(buffer, width=20, height=5, frame_time=16.5)
+
+        bottom_line = result[-1]
+        assert "16.5ms" in bottom_line
+
+    def test_render_border_crops_content_to_fit(self):
+        """render_border crops content to fit within borders."""
+        from engine.display import render_border
+
+        # Buffer larger than viewport
+        buffer = ["x" * 100] * 50
+        result = render_border(buffer, width=20, height=10)
+
+        # Result shrinks to fit viewport
+        assert len(result) == 10
+        for line in result[1:-1]:  # Skip border lines
+            assert len(line) == 20
+
+    def test_render_border_preserves_content(self):
+        """render_border preserves content within borders."""
+        from engine.display import render_border
+
+        buffer = ["hello world", "test line"]
+        result = render_border(buffer, width=20, height=5)
+
+        # Content should appear in the middle rows
+        content_lines = result[1:-1]
+        assert any("hello world" in line for line in content_lines)
+
+    def test_render_border_with_small_buffer(self):
+        """render_border handles buffers smaller than viewport."""
+        from engine.display import render_border
+
+        buffer = ["hi"]
+        result = render_border(buffer, width=20, height=10)
+
+        # Should still produce full viewport with padding
+        assert len(result) == 10
+        # All lines should be full width
+        for line in result:
+            assert len(line) == 20
+
+
 class TestMultiDisplay:
     """Tests for MultiDisplay class."""