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

engine/display/backends/terminal.py

@@ -22,6 +22,7 @@ class TerminalDisplay:
         self.target_fps = target_fps
         self._frame_period = 1.0 / target_fps if target_fps > 0 else 0
         self._last_frame_time = 0.0
+        self._cached_dimensions: tuple[int, int] | None = None
 
     def init(self, width: int, height: int, reuse: bool = False) -> None:
         """Initialize display with dimensions.
@@ -62,14 +63,26 @@ class TerminalDisplay:
     def get_dimensions(self) -> tuple[int, int]:
         """Get current terminal dimensions.
 
+        Returns cached dimensions to avoid querying terminal every frame,
+        which can cause inconsistent results. Dimensions are only refreshed
+        when they actually change.
+
         Returns:
             (width, height) in character cells
         """
         try:
             term_size = os.get_terminal_size()
-            return (term_size.columns, term_size.lines)
+            new_dims = (term_size.columns, term_size.lines)
         except OSError:
-            return (self.width, self.height)
+            new_dims = (self.width, self.height)
+
+        # Only update cached dimensions if they actually changed
+        if self._cached_dimensions is None or self._cached_dimensions != new_dims:
+            self._cached_dimensions = new_dims
+            self.width = new_dims[0]
+            self.height = new_dims[1]
+
+        return self._cached_dimensions
 
     def show(self, buffer: list[str], border: bool = False) -> None:
         import sys
@@ -103,10 +116,9 @@ class TerminalDisplay:
         if border:
             buffer = render_border(buffer, self.width, self.height, fps, frame_time)
 
-        # Clear screen and home cursor before each frame
-        from engine.terminal import CLR
-
-        output = CLR + "".join(buffer)
+        # Write buffer with cursor home + erase down to avoid flicker
+        # \033[H = cursor home, \033[J = erase from cursor to end of screen
+        output = "\033[H\033[J" + "".join(buffer)
         sys.stdout.buffer.write(output.encode())
         sys.stdout.flush()
         elapsed_ms = (time.perf_counter() - t0) * 1000
@@ -124,3 +136,11 @@ class TerminalDisplay:
         from engine.terminal import CURSOR_ON
 
         print(CURSOR_ON, end="", flush=True)
+
+    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