NordVPN macOS DNS Plan

Add macOS DNS state file support under ~/.nordvpn-client.
Implement helpers to enumerate eligible physical services and snapshot existing DNS/search-domain settings.
Implement helpers to apply NordVPN DNS only to eligible physical services.
Implement helpers to restore previous DNS/search-domain settings on disconnect or failed connect.
Add unit tests for service filtering and DNS state transitions.
Update skill/docs to explain macOS physical-service DNS management.
Sync the installed workspace copy.
Run tests and non-destructive verification.
Commit and push.