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

engine/display/backends/null.py

@@ -9,11 +9,16 @@ class NullDisplay:
     """Headless/null display - discards all output.
 
     This display does nothing - useful for headless benchmarking
-    or when no display output is needed.
+    or when no display output is needed. Captures last buffer
+    for testing purposes.
     """
 
     width: int = 80
     height: int = 24
+    _last_buffer: list[str] | None = None
+
+    def __init__(self):
+        self._last_buffer = None
 
     def init(self, width: int, height: int, reuse: bool = False) -> None:
         """Initialize display with dimensions.
@@ -25,10 +30,12 @@ class NullDisplay:
         """
         self.width = width
         self.height = height
+        self._last_buffer = None
 
     def show(self, buffer: list[str], border: bool = False) -> None:
         from engine.display import get_monitor
 
+        self._last_buffer = buffer
         monitor = get_monitor()
         if monitor:
             t0 = time.perf_counter()
@@ -49,3 +56,11 @@ class NullDisplay:
             (width, height) in character cells
         """
         return (self.width, self.height)
+
+    def is_quit_requested(self) -> bool:
+        """Check if quit was requested (optional protocol method)."""
+        return False
+
+    def clear_quit_request(self) -> None:
+        """Clear quit request (optional protocol method)."""
+        pass