fix(release): use macOS runner to cross-compile all targets

Linux runners can't cross-compile for macOS (missing SDK headers for
vendored OpenSSL). macOS runners can cross-compile Linux via
cargo-zigbuild and build macOS targets natively.

- Switch runner from ubuntu-latest to macos-latest
- Update zig download for macOS aarch64
- Add --parallelism 4 to goreleaser

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>