Both arches now publish from CI: the existing amd64 track plus a new
build-binary-arm64 (native ubuntu-24.04-arm, free for public repos, reuses
release-linux-aarch64's cargo cache) -> snap-arm64 that dumps the prebuilt
aarch64 binary with packaging/snap-slint/snapcraft-arm64.yaml. This CI upload
is the sole publish path; Launchpad / "Build from GitHub" is not used.
cc
