fix: sample playback (don't reload every keypress) + add sync/poweroff to busybox
- notepat: only call loadData when switching to a different sample,
not on every key press (was swapping buffer mid-playback = silence)
- docker-build: add sync, poweroff, reboot, halt, mknod busybox symlinks
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>