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/api/types.go

@@ -409,7 +409,7 @@ type LUNMap map[uint64]*SCSILu
 
 type UnmapBlockDescriptor struct {
 	Offset uint64
-	TL     uint32
+	TL     uint64
 }
 
 type ReaderWriterAt interface {