test: add comprehensive unit tests for core components

- tests/test_canvas.py: 33 tests for Canvas (2D rendering surface)
- tests/test_firehose.py: 5 tests for FirehoseEffect
- tests/test_pipeline_order.py: 3 tests for execution order verification
- tests/test_renderer.py: 22 tests for ANSI parsing and PIL rendering

These tests provide solid coverage for foundational modules.

tests/test_renderer.py

@@ -0,0 +1,164 @@
+"""
+Unit tests for engine.display.renderer module.
+
+Tests ANSI parsing and PIL rendering utilities.
+"""
+
+import pytest
+
+try:
+    from PIL import Image
+
+    PIL_AVAILABLE = True
+except ImportError:
+    PIL_AVAILABLE = False
+
+from engine.display.renderer import ANSI_COLORS, parse_ansi, render_to_pil
+
+
+class TestParseANSI:
+    """Tests for parse_ansi function."""
+
+    def test_plain_text(self):
+        """Plain text without ANSI codes returns single token."""
+        tokens = parse_ansi("Hello World")
+        assert len(tokens) == 1
+        assert tokens[0][0] == "Hello World"
+        # Check default colors
+        assert tokens[0][1] == (204, 204, 204)  # fg
+        assert tokens[0][2] == (0, 0, 0)  # bg
+        assert tokens[0][3] is False  # bold
+
+    def test_empty_string(self):
+        """Empty string returns single empty token."""
+        tokens = parse_ansi("")
+        assert tokens == [("", (204, 204, 204), (0, 0, 0), False)]
+
+    def test_reset_code(self):
+        """Reset code (ESC[0m) restores defaults."""
+        tokens = parse_ansi("\x1b[31mRed\x1b[0mNormal")
+        assert len(tokens) == 2
+        assert tokens[0][0] == "Red"
+        # Red fg should be ANSI_COLORS[1]
+        assert tokens[0][1] == ANSI_COLORS[1]
+        assert tokens[1][0] == "Normal"
+        assert tokens[1][1] == (204, 204, 204)  # back to default
+
+    def test_bold_code(self):
+        """Bold code (ESC[1m) sets bold flag."""
+        tokens = parse_ansi("\x1b[1mBold")
+        assert tokens[0][3] is True
+
+    def test_bold_off_code(self):
+        """Bold off (ESC[22m) clears bold."""
+        tokens = parse_ansi("\x1b[1mBold\x1b[22mNormal")
+        assert tokens[0][3] is True
+        assert tokens[1][3] is False
+
+    def test_4bit_foreground_colors(self):
+        """4-bit foreground colors (30-37, 90-97) work."""
+        # Test normal red (31)
+        tokens = parse_ansi("\x1b[31mRed")
+        assert tokens[0][1] == ANSI_COLORS[1]  # color 1 = red
+
+        # Test bright cyan (96) - maps to index 14 (bright cyan)
+        tokens = parse_ansi("\x1b[96mCyan")
+        assert tokens[0][1] == ANSI_COLORS[14]  # bright cyan
+
+    def test_4bit_background_colors(self):
+        """4-bit background colors (40-47, 100-107) work."""
+        # Green bg = 42
+        tokens = parse_ansi("\x1b[42mText")
+        assert tokens[0][2] == ANSI_COLORS[2]  # color 2 = green
+
+        # Bright magenta bg = 105
+        tokens = parse_ansi("\x1b[105mText")
+        assert tokens[0][2] == ANSI_COLORS[13]  # bright magenta (13)
+
+    def test_multiple_ansi_codes_in_sequence(self):
+        """Multiple codes in one escape sequence are parsed."""
+        tokens = parse_ansi("\x1b[1;31;42mBold Red on Green")
+        assert tokens[0][0] == "Bold Red on Green"
+        assert tokens[0][3] is True  # bold
+        assert tokens[0][1] == ANSI_COLORS[1]  # red fg
+        assert tokens[0][2] == ANSI_COLORS[2]  # green bg
+
+    def test_nested_ansi_sequences(self):
+        """Multiple separate ANSI sequences are tokenized correctly."""
+        text = "\x1b[31mRed\x1b[32mGreen\x1b[0mNormal"
+        tokens = parse_ansi(text)
+        assert len(tokens) == 3
+        assert tokens[0][0] == "Red"
+        assert tokens[1][0] == "Green"
+        assert tokens[2][0] == "Normal"
+
+    def test_interleaved_text_and_ansi(self):
+        """Text before and after ANSI codes is tokenized."""
+        tokens = parse_ansi("Pre\x1b[31mRedPost")
+        assert len(tokens) == 2
+        assert tokens[0][0] == "Pre"
+        assert tokens[1][0] == "RedPost"
+        assert tokens[1][1] == ANSI_COLORS[1]
+
+    def test_all_standard_4bit_colors(self):
+        """All 4-bit color indices (0-15) map to valid RGB."""
+        for i in range(16):
+            tokens = parse_ansi(f"\x1b[{i}mX")
+            # Should be a defined color or default fg
+            fg = tokens[0][1]
+            valid = fg in ANSI_COLORS.values() or fg == (204, 204, 204)
+            assert valid, f"Color {i} produced invalid fg {fg}"
+
+    def test_unknown_code_ignored(self):
+        """Unknown numeric codes are ignored, keep current style."""
+        tokens = parse_ansi("\x1b[99mText")
+        # 99 is not recognized, should keep previous state (defaults)
+        assert tokens[0][1] == (204, 204, 204)
+
+
+@pytest.mark.skipif(not PIL_AVAILABLE, reason="PIL not available")
+class TestRenderToPIL:
+    """Tests for render_to_pil function (requires PIL)."""
+
+    def test_renders_plain_text(self):
+        """Plain buffer renders as image."""
+        buffer = ["Hello"]
+        img = render_to_pil(buffer, width=10, height=1)
+        assert isinstance(img, Image.Image)
+        assert img.mode == "RGBA"
+
+    def test_renders_with_ansi_colors(self):
+        """Buffer with ANSI colors renders correctly."""
+        buffer = ["\x1b[31mRed\x1b[0mNormal"]
+        img = render_to_pil(buffer, width=20, height=1)
+        assert isinstance(img, Image.Image)
+
+    def test_multi_line_buffer(self):
+        """Multiple lines render with correct height."""
+        buffer = ["Line1", "Line2", "Line3"]
+        img = render_to_pil(buffer, width=10, height=3)
+        # Height should be approximately 3 * cell_height (18-2 padding)
+        assert img.height > 0
+
+    def test_clipping_to_height(self):
+        """Buffer longer than height is clipped."""
+        buffer = ["Line1", "Line2", "Line3", "Line4"]
+        img = render_to_pil(buffer, width=10, height=2)
+        # Should only render 2 lines
+        assert img.height < img.width * 2  # roughly 2 lines tall
+
+    def test_cell_dimensions_respected(self):
+        """Custom cell_width and cell_height are used."""
+        buffer = ["Test"]
+        img = render_to_pil(buffer, width=5, height=1, cell_width=20, cell_height=25)
+        assert img.width == 5 * 20
+        assert img.height == 25
+
+    def test_font_fallback_on_invalid(self):
+        """Invalid font path falls back to default font."""
+        buffer = ["Test"]
+        # Should not crash with invalid font path
+        img = render_to_pil(
+            buffer, width=5, height=1, font_path="/nonexistent/font.ttf"
+        )
+        assert isinstance(img, Image.Image)