Default stays linux-x64 (the only architecture built today).
Override on the command line to target arm64 (or any other
RID supported by Avalonia 12 on Linux):
make deb POSTIT_RUNTIME=linux-arm64
The Makefile passes POSTIT_RUNTIME through to dpkg-buildpackage,
which propagates it to debian/rules, which threads it into
dotnet publish --runtime.
Note: the .deb is mono-architecture. For a multi-arch .deb,
the build must run twice (once per RID) and the resulting .debs
can be combined with dpkg-gencontrol / Multi-Arch: same.
Update README accordingly.
