release: one 'make release' for the three target binaries

Streamline the release into a single target that stages all three binaries
in dist/ with a SHA256SUMS:

  linux x86_64  }  cross-built here with 'zig cc -target ... -s', static musl
  linux aarch64 }  (runs on any distro), packaged as a .tar.gz with the binary
                   + README + LICENSE + THIRD-PARTY-LICENSES
  macos arm64      handed to 'macos-dist', which signs/notarizes on the builder

zig is already the compiler, so the two linux cross-builds need no extra
toolchain -- just the vendored libghostty-vt rebuilt per -Dtarget. Verified
both: static, stripped (~2MB), and both run (aarch64 under qemu-user).

Factor the version ladder out of the Makefile into contrib/version, and call
it from both Makefile and Makefile.macos, so the linux tarballs, the signed
zip and the binary's own --version are all named from the same tag instead
of Makefile.macos using its own 'git describe --always' scheme (which would
have named the mac artifact after a bare commit hash while linux used the
tag).

  make release-linux            # just the two linux tarballs
  make release REF=<tag>        # all three + SHA256SUMS

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