- Add scanActive, scanCancel, scanPaused, scanPauseCh fields to Library struct
- Create scan_control.go with CancelScan, PauseScan, ResumeScan, IsScanActive, IsScanPaused
- Thread per-scan scanCtx through walk and worker pipeline
- Add waitIfPaused checkpoint before each worker extraction
- Skip orphan cleanup and variant generation on cancelled scan
- Emit LibraryScanCancelled instead of LibraryScanComplete when cancelled
