build(release): ship per-arch macOS tarballs instead of universal

[brianjfox]

Summary

Replaces the single universal (arm64 + x86_64) macOS release tarball with two per-architecture tarballs, gnash-X.Y.Z-macos-arm64.tar.gz and gnash-X.Y.Z-macos-x86_64.tar.gz, each with its own .sha256. A universal binary carries both slices, so each direct downloader was paying for an architecture they can't run; the per-arch tarballs are roughly half the size (~770–850 KB gzipped vs ~1.5 MB).

Homebrew is unaffected: the formula installs from the source tarball, and bottles are already selected per platform tag by brew itself.

Changes

• scripts/release.sh step 3 now loops over arm64 and x86_64, building each in its own build-release-<arch> tree (already covered by .gitignore's build-*/), verifying the slice with lipo -archs, and packaging per-arch tarballs. Idempotency is preserved per architecture.
• Step 4 uploads all four assets via a new ASSETS array.
• Generated release notes and the README now point users at the tarball for their architecture.

Testing

• bash -n passes; the --help text range still renders correctly.
• Ran the new step-3 loop end to end against a scratch staging dir: both arches build (x86_64 cross-compiles fine on Apple Silicon), lipo verification passes, tarballs package with the license/README extras, and a re-run skips both (idempotent).
• Extracted the arm64 tarball and ran the binary: works.

🤖 Generated with Claude Code