A garbage or out-of-range port silently fell back to 22 and connected
to the wrong place; fail loud instead.

rsh -keygen printed the pubkey but wrote the key pair into a directory,
so generating a key briefly left the plaintext private key on flash,
contradicting the documented invariant that it only ever exists in
memory. -keygen now emits the private key PEM on stdout and nothing
else; the app encrypts it immediately and derives the public key via
-pubkey, reusing the validated import path. Keygen failures now surface
as an error dialog instead of crashing the app from a bare thread.

A small Android app for one-way folder backup, a KISS alternative to
Syncthing. It bundles rsync (built from pinned source via the NDK) and
a pure-Go SSH transport, both shipped in the APK as lib*.so and run from
the native library directory.

rsend pins the host key, stores the ed25519 identity Keystore-encrypted,
pushes each folder additively or as a mirror, and runs on demand or on a
WiFi-only schedule. The build is self-contained and reproducible: make
setup provisions the toolchain, make builds rsync, rsh, and the APK.