Fix openssl@3 installation - use pre-installed version

ci(release): replace sha256sum with shasum -a 256 in release workflow

ci: unify macOS builds into matrix, add caching, package artifacts, a…

…nd update release flow

- Convert separate macOS ARM64/x86_64 jobs into a single matrix-based build-and-test job
- Cache Zig runtime, Homebrew cache and Zig build artifacts (.zig-cache) to speed up runs
- Add Zig PATH handling and conditional Zig setup when cache miss
- Verify vpnclient binary before running tests
- In release workflow: make macOS builds matrix-driven, produce per-arch vpnclient and lib packages and generate SHA256 checksums
- Add create-universal-binary job to combine arm64/x86_64 vpnclient into a universal binary and package it
- Update create-release to depend on new jobs, include universal artifact, and generate release notes for downloads/checksums

Include vpnclient executable in release

ci(release): enable create-release job for macOS-only releases

Add a Create GitHub Release job that runs after build-macos, downloads the macOS artifact
and publishes a GitHub release containing the macos-arm64 archive. Keep Linux/Windows
jobs commented out and simplify their commented zig build lines to use plain `zig build`.