4.1: Add macos-14 arm64 runner to CI matrix #14

Add macos-14 (Apple Silicon) to .github/workflows/main.yml:

- platform: arm64
  os: macos-14
  triplet: arm64-osx

• macOS job builds successfully on every push
• Build time is acceptable (vcpkg cache warm path)
• Windows job unaffected

See MACOS_PORT.md §Phase 4 · 4.1