refactor(display): extract shared rendering logic into renderer.py

- Add renderer.py with parse_ansi(), get_default_font_path(), render_to_pil()
- Update KittyDisplay and SixelDisplay to use shared renderer
- Enhance parse_ansi to handle full ANSI color codes (4-bit, 256-color)
- Update tests to use shared renderer functions

tests/test_sixel.py

@@ -69,40 +69,45 @@ class TestSixelAnsiParsing:
 
     def test_parse_empty_string(self):
         """handles empty string."""
-        from engine.display.backends.sixel import _parse_ansi
+        from engine.display.renderer import parse_ansi
 
-        result = _parse_ansi("")
+        result = parse_ansi("")
         assert len(result) > 0
 
     def test_parse_plain_text(self):
         """parses plain text without ANSI codes."""
-        from engine.display.backends.sixel import _parse_ansi
+        from engine.display.renderer import parse_ansi
 
-        result = _parse_ansi("hello world")
+        result = parse_ansi("hello world")
         assert len(result) == 1
         text, fg, bg, bold = result[0]
         assert text == "hello world"
 
     def test_parse_with_color_codes(self):
         """parses ANSI color codes."""
-        from engine.display.backends.sixel import _parse_ansi
+        from engine.display.renderer import parse_ansi
 
-        result = _parse_ansi("\033[31mred\033[0m")
-        assert len(result) == 2
+        result = parse_ansi("\033[31mred\033[0m")
+        assert len(result) == 1
+        assert result[0][0] == "red"
+        assert result[0][1] == (205, 49, 49)
 
     def test_parse_with_bold(self):
         """parses bold codes."""
-        from engine.display.backends.sixel import _parse_ansi
+        from engine.display.renderer import parse_ansi
 
-        result = _parse_ansi("\033[1mbold\033[0m")
-        assert len(result) == 2
+        result = parse_ansi("\033[1mbold\033[0m")
+        assert len(result) == 1
+        assert result[0][0] == "bold"
+        assert result[0][3] is True
 
     def test_parse_256_color(self):
         """parses 256 color codes."""
-        from engine.display.backends.sixel import _parse_ansi
+        from engine.display.renderer import parse_ansi
 
-        result = _parse_ansi("\033[38;5;196mred\033[0m")
-        assert len(result) == 2
+        result = parse_ansi("\033[38;5;196mred\033[0m")
+        assert len(result) == 1
+        assert result[0][0] == "red"
 
 
 class TestSixelEncoding: