chore(display): add debug logging to NullDisplay for development

- Print first few frames periodically to aid debugging
- Remove obsolete design doc

This helps inspect buffer contents when running headless tests.

engine/display/backends/null.py

@@ -33,6 +33,8 @@ class NullDisplay:
         self._last_buffer = None
 
     def show(self, buffer: list[str], border: bool = False) -> None:
+        import sys
+
         from engine.display import get_monitor, render_border
 
         # Get FPS for border (if available)
@@ -52,6 +54,26 @@ class NullDisplay:
             buffer = render_border(buffer, self.width, self.height, fps, frame_time)
 
         self._last_buffer = buffer
+
+        # For debugging: print first few frames to stdout
+        if hasattr(self, "_frame_count"):
+            self._frame_count += 1
+        else:
+            self._frame_count = 0
+
+        # Only print first 5 frames or every 10th frame
+        if self._frame_count <= 5 or self._frame_count % 10 == 0:
+            sys.stdout.write("\n" + "=" * 80 + "\n")
+            sys.stdout.write(
+                f"Frame {self._frame_count} (buffer height: {len(buffer)})\n"
+            )
+            sys.stdout.write("=" * 80 + "\n")
+            for i, line in enumerate(buffer[:30]):  # Show first 30 lines
+                sys.stdout.write(f"{i:2}: {line}\n")
+            if len(buffer) > 30:
+                sys.stdout.write(f"... ({len(buffer) - 30} more lines)\n")
+            sys.stdout.flush()
+
         if monitor:
             t0 = time.perf_counter()
             chars_in = sum(len(line) for line in buffer)