Refine start menu difficulty and preview workflow

AGENTS.md

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+# Project Guidelines
+
+## UI Preview Tool
+
+When editing the start menu, pause menu, or level intro UI, generate a real preview image before judging layout changes.
+
+Use [tools/render_menu_preview.py](tools/render_menu_preview.py) instead of relying on mental layout or ad-hoc screenshots. The tool renders the actual SDL scene and saves a PNG from the real renderer.
+
+Typical command:
+
+```bash
+/home/enne2/dev/mice/.venv/bin/python tools/render_menu_preview.py \
+  --output /tmp/mice_start_preview.png \
+  --screen start \
+  --difficulty normal \
+  --resolution 1280x720
+```
+
+Supported screens:
+
+- `start`
+- `pause`
+- `level_intro`
+
+Useful flags:
+
+- `--difficulty easy|normal|hard`
+- `--resolution WIDTHxHEIGHT`
+- `--output /path/to/file.png`
+- `--seed N` for deterministic previews
+- `--animation-ms N` to choose the GIF frame timestamp for the start menu
+
+Workflow when touching menu layout:
+
+1. Edit the menu code.
+2. Run the preview tool for the relevant screen.
+3. Inspect the generated PNG.
+4. Iterate until spacing and readability are correct.
+
+The preview tool is intended for fast visual feedback and should be preferred before launching a full interactive game session for menu-only changes.