Security and robustness hardening:
- Harden private WallpaperExtensionKit runtime bridging — guarded casts, bounds-checked runtime writes, and a startup compatibility self-check (#8)
- Validate library metadata to prevent path traversal (#9)
- Validate the XPC caller's code signature before accepting wallpaper-host connections (#11)
- Clean up renderer layers when playback stops (#5)
- Rotate the extension log so it can't grow unbounded (#4)