ci(release): publish each platform asset as soon as its build finishes

Previously the single 'release' job depended on all 6 builds, so nothing
appeared on the GitHub release until the slowest job (x86_64 windows/linux,
~30m) completed. Now a lightweight 'create-release' job publishes the release
up front, each build job attaches its own asset via 'gh release upload
--clobber' the moment it finishes, and a 'finalize' job adds SHA256SUMS +
Homebrew/AUR once everything is done.

Effect: the macOS aarch64 binary is downloadable ~15m in instead of ~30m,
independent of the Windows/Linux x86_64 long pole.