fix: resolve terminal display wobble and effect dimension stability

- Fix TerminalDisplay: add screen clear each frame (cursor home + erase down)
- Fix CameraStage: use set_canvas_size instead of read-only viewport properties
- Fix Glitch effect: preserve visible line lengths, remove cursor positioning
- Fix Fade effect: return original line when fade=0 instead of empty string
- Fix Noise effect: use input line length instead of terminal_width
- Remove HUD effect from all presets (redundant with border FPS display)
- Add regression tests for effect dimension stability
- Add docs/ARCHITECTURE.md with Mermaid diagrams
- Add mise tasks: diagram-ascii, diagram-validate, diagram-check
- Move markdown docs to docs/ (ARCHITECTURE, Refactor, hardware specs)
- Remove redundant requirements files (use pyproject.toml)
- Add *.dot and *.png to .gitignore

Closes #25

tests/test_pipeline.py

@@ -483,7 +483,7 @@ class TestPipelineParams:
         assert params.source == "headlines"
         assert params.display == "terminal"
         assert params.camera_mode == "vertical"
-        assert params.effect_order == ["noise", "fade", "glitch", "firehose", "hud"]
+        assert params.effect_order == ["noise", "fade", "glitch", "firehose"]
 
     def test_effect_config(self):
         """PipelineParams effect config methods work."""
@@ -634,6 +634,33 @@ class TestStageAdapters:
         assert "camera" in stage.capabilities
         assert "render.output" in stage.dependencies  # Depends on rendered content
 
+    def test_camera_stage_does_not_error_on_process(self):
+        """CameraStage.process should not error when setting viewport.
+
+        Regression test: Previously CameraStage tried to set viewport_width
+        and viewport_height as writable properties, but they are computed
+        from canvas_size / zoom. This caused an AttributeError each frame.
+        """
+        from engine.camera import Camera, CameraMode
+        from engine.pipeline.adapters import CameraStage
+        from engine.pipeline.core import PipelineContext
+        from engine.pipeline.params import PipelineParams
+
+        camera = Camera(mode=CameraMode.FEED)
+        stage = CameraStage(camera, name="vertical")
+
+        ctx = PipelineContext()
+        ctx.params = PipelineParams(viewport_width=80, viewport_height=24)
+
+        buffer = ["line" + str(i) for i in range(24)]
+
+        # This should not raise AttributeError
+        result = stage.process(buffer, ctx)
+
+        # Should return the buffer (unchanged for FEED mode)
+        assert result is not None
+        assert len(result) == 24
+
 
 class TestDataSourceStage:
     """Tests for DataSourceStage adapter."""