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_display.py

@@ -2,6 +2,7 @@
 Tests for engine.display module.
 """
 
+import sys
 from unittest.mock import MagicMock
 
 from engine.display import DisplayRegistry, NullDisplay, TerminalDisplay
@@ -115,6 +116,83 @@ class TestTerminalDisplay:
         display = TerminalDisplay()
         display.cleanup()
 
+    def test_get_dimensions_returns_cached_value(self):
+        """get_dimensions returns cached dimensions for stability."""
+        display = TerminalDisplay()
+        display.init(80, 24)
+
+        # First call should set cache
+        d1 = display.get_dimensions()
+        assert d1 == (80, 24)
+
+    def test_show_clears_screen_before_each_frame(self):
+        """show clears previous frame to prevent visual wobble.
+
+        Regression test: Previously show() didn't clear the screen,
+        causing old content to remain and creating visual wobble.
+        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)
+
+        buffer = ["line1", "line2", "line3"]
+
+        fake_buffer = BytesIO()
+        fake_stdout = MagicMock()
+        fake_stdout.buffer = fake_buffer
+        with patch.object(sys, "stdout", fake_stdout):
+            display.show(buffer)
+
+        output = fake_buffer.getvalue().decode("utf-8")
+        assert output.startswith("\033[H\033[J"), (
+            f"Output should start with clear sequence, got: {repr(output[:20])}"
+        )
+
+    def test_show_clears_screen_on_subsequent_frames(self):
+        """show clears screen on every frame, not just the first.
+
+        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)
+        display.init(80, 24)
+
+        buffer = ["line1", "line2"]
+
+        for i in range(3):
+            fake_buffer = BytesIO()
+            fake_stdout = MagicMock()
+            fake_stdout.buffer = fake_buffer
+            with patch.object(sys, "stdout", fake_stdout):
+                display.show(buffer)
+
+            output = fake_buffer.getvalue().decode("utf-8")
+            assert output.startswith("\033[H\033[J"), (
+                f"Frame {i} should start with clear sequence"
+            )
+
+    def test_get_dimensions_stable_across_rapid_calls(self):
+        """get_dimensions should not fluctuate when called rapidly.
+
+        This test catches the bug where os.get_terminal_size() returns
+        inconsistent values, causing visual wobble.
+        """
+        display = TerminalDisplay()
+        display.init(80, 24)
+
+        # Get dimensions 10 times rapidly (simulating frame loop)
+        dims = [display.get_dimensions() for _ in range(10)]
+
+        # All should be the same - this would fail if os.get_terminal_size()
+        # returns different values each call
+        assert len(set(dims)) == 1, f"Dimensions should be stable, got: {set(dims)}"
+
 
 class TestNullDisplay:
     """Tests for NullDisplay class."""
@@ -141,6 +219,27 @@ class TestNullDisplay:
         display = NullDisplay()
         display.cleanup()
 
+    def test_show_stores_last_buffer(self):
+        """show stores last buffer for testing inspection."""
+        display = NullDisplay()
+        display.init(80, 24)
+
+        buffer = ["line1", "line2", "line3"]
+        display.show(buffer)
+
+        assert display._last_buffer == buffer
+
+    def test_show_tracks_last_buffer_across_calls(self):
+        """show updates last_buffer on each call."""
+        display = NullDisplay()
+        display.init(80, 24)
+
+        display.show(["first"])
+        assert display._last_buffer == ["first"]
+
+        display.show(["second"])
+        assert display._last_buffer == ["second"]
+
 
 class TestMultiDisplay:
     """Tests for MultiDisplay class."""