Add the new ppa:rsyncproject/rsync-latest (development snapshots rebuilt
from git master) alongside the existing stable PPA in INSTALL.md and the
download page. Notes that snapshot versions (3.5.0~git...) sort below the
matching stable release, so the two PPAs can coexist without a stable
release being silently replaced by a snapshot.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
