fix UNMAP data corruption by implementing block zeroing

The UNMAP command was a no-op in all backing stores, causing unmapped
blocks to retain stale data instead of returning zeros per SCSI spec.

- Implement Unmap in FileBackingStore to zero out unmapped blocks
- Implement Unmap in IOUringBackingStore to zero out unmapped blocks
- Enable Unmap in RemBackingStore (was commented out)
- Change UnmapBlockDescriptor.TL from uint32 to uint64 to prevent
  integer overflow when converting block count to byte length with
  large block shifts

Fixes #119

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/scsi/sbc.go

@@ -320,7 +320,7 @@ func SBCUnmap(host int, cmd *api.SCSICommand) api.SAMStat {
 		num := binary.BigEndian.Uint32(cmd.OutSDBBuffer.Buffer[off+8 : off+12])
 		blockDescs = append(blockDescs, api.UnmapBlockDescriptor{
 			Offset: lba << cmd.Device.BlockShift,
-			TL:     num << cmd.Device.BlockShift,
+			TL:     uint64(num) << cmd.Device.BlockShift,
 		})
 	}