fix: control macOS media keys without synthetic events

[matteoveglia]

Summary

• control default-output volume and mute through CoreAudio
• send play/pause, previous, and next to the active Now Playing client through MediaRemote
• retain the existing synthetic media-key path as a fallback when either route is unavailable

Validation

• dotnet build Hydra --configuration Release --no-restore --nologo
• dotnet test Tests/Tests.csproj --configuration Release --no-restore --filter "FullyQualifiedName~MacMediaRemoteControllerTests|FullyQualifiedName~MacKeyResolverTests|FullyQualifiedName~MacModifierMappingTests" --no-logo
• dotnet publish Hydra --configuration Release --runtime osx-arm64 --self-contained --no-restore --nologo
• live validation on macOS 27: volume, mute, play/pause, previous, and next

Notes

• CoreAudio is public; MediaRemote is private and dynamically resolved, so unsupported systems keep the legacy fallback.