docs: the Termux home now has a working aarch64 Android SDK, so record how to build here, how the debug build differs from the release one (separate package, own data directory, so it cannot touch the installed launcher's config), and warn about the destructive uninstall path. scripts/ondevice.sh wraps the loop: build, install over the debug package, launch, follow logcat, read the newest crash report, and build the signed release APK.