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_pipeline_order.py

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+"""Tests for pipeline execution order verification."""
+
+from unittest.mock import MagicMock
+
+import pytest
+
+from engine.pipeline import Pipeline, Stage, discover_stages
+from engine.pipeline.core import DataType
+
+
+@pytest.fixture(autouse=True)
+def reset_registry():
+    """Reset stage registry before each test."""
+    from engine.pipeline.registry import StageRegistry
+
+    StageRegistry._discovered = False
+    StageRegistry._categories.clear()
+    StageRegistry._instances.clear()
+    discover_stages()
+    yield
+    StageRegistry._discovered = False
+    StageRegistry._categories.clear()
+    StageRegistry._instances.clear()
+
+
+def _create_mock_stage(name: str, category: str, capabilities: set, dependencies: set):
+    """Helper to create a mock stage."""
+    mock = MagicMock(spec=Stage)
+    mock.name = name
+    mock.category = category
+    mock.stage_type = category
+    mock.render_order = 0
+    mock.is_overlay = False
+    mock.inlet_types = {DataType.ANY}
+    mock.outlet_types = {DataType.TEXT_BUFFER}
+    mock.capabilities = capabilities
+    mock.dependencies = dependencies
+    mock.process = lambda data, ctx: data
+    mock.init = MagicMock(return_value=True)
+    mock.cleanup = MagicMock()
+    mock.is_enabled = MagicMock(return_value=True)
+    mock.set_enabled = MagicMock()
+    mock._enabled = True
+    return mock
+
+
+def test_pipeline_execution_order_linear():
+    """Verify stages execute in linear order based on dependencies."""
+    pipeline = Pipeline()
+    pipeline.build(auto_inject=False)
+
+    source = _create_mock_stage("source", "source", {"source"}, set())
+    render = _create_mock_stage("render", "render", {"render"}, {"source"})
+    effect = _create_mock_stage("effect", "effect", {"effect"}, {"render"})
+    display = _create_mock_stage("display", "display", {"display"}, {"effect"})
+
+    pipeline.add_stage("source", source, initialize=False)
+    pipeline.add_stage("render", render, initialize=False)
+    pipeline.add_stage("effect", effect, initialize=False)
+    pipeline.add_stage("display", display, initialize=False)
+
+    pipeline._rebuild()
+
+    assert pipeline.execution_order == [
+        "source",
+        "render",
+        "effect",
+        "display",
+    ]
+
+
+def test_pipeline_effects_chain_order():
+    """Verify effects execute in config order when chained."""
+    pipeline = Pipeline()
+    pipeline.build(auto_inject=False)
+
+    # Source and render
+    source = _create_mock_stage("source", "source", {"source"}, set())
+    render = _create_mock_stage("render", "render", {"render"}, {"source"})
+
+    # Effects chain: effect_a → effect_b → effect_c
+    effect_a = _create_mock_stage("effect_a", "effect", {"effect_a"}, {"render"})
+    effect_b = _create_mock_stage("effect_b", "effect", {"effect_b"}, {"effect_a"})
+    effect_c = _create_mock_stage("effect_c", "effect", {"effect_c"}, {"effect_b"})
+
+    # Display
+    display = _create_mock_stage("display", "display", {"display"}, {"effect_c"})
+
+    for stage in [source, render, effect_a, effect_b, effect_c, display]:
+        pipeline.add_stage(stage.name, stage, initialize=False)
+
+    pipeline._rebuild()
+
+    effect_names = [
+        name for name in pipeline.execution_order if name.startswith("effect_")
+    ]
+    assert effect_names == ["effect_a", "effect_b", "effect_c"]
+
+
+def test_pipeline_overlay_executes_after_regular_effects():
+    """Overlay stages should execute after all regular effects."""
+    pipeline = Pipeline()
+    pipeline.build(auto_inject=False)
+
+    effect = _create_mock_stage("effect1", "effect", {"effect1"}, {"render"})
+    overlay = _create_mock_stage("overlay_test", "overlay", {"overlay"}, {"effect1"})
+    display = _create_mock_stage("display", "display", {"display"}, {"overlay"})
+
+    for stage in [effect, overlay, display]:
+        pipeline.add_stage(stage.name, stage, initialize=False)
+
+    pipeline._rebuild()
+
+    names = pipeline.execution_order
+    idx_effect = names.index("effect1")
+    idx_overlay = names.index("overlay_test")
+    idx_display = names.index("display")
+    assert idx_effect < idx_overlay < idx_display