4.1: Add macos-14 arm64 runner to CI matrix

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

```yaml
- 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