add end-to-end IO benchmarks and fix pprof-identified hotspots

Add comprehensive benchmark suite (io_bench_test.go):
- BenchmarkEndToEndRead/Write: full SCSI stack (512B to 256KB)
- BenchmarkEndToEndReadParallel/WriteParallel: concurrent IO
- BenchmarkFileBackingStoreRead/Write: isolated backing store

pprof-guided optimizations:
- Guard hot-path log.Debugf with log.GetLevel() check in scsi.go,
  sbc.go, backingstore.go — eliminates 22% CPU overhead from logrus
  Entry allocation even when debug logging is disabled
- Add FileBackingStore.ReadAt for zero-copy reads directly into
  caller's buffer, bypassing Read()'s per-call make([]byte, tl)
- Use ReadAt via interface assertion in bsPerformCommand to read
  directly into InSDBBuffer, eliminating allocation + copy

Results (256KB reads): +42% throughput, allocs reduced from 10 to 5

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

pkg/scsi/scsi.go

@@ -98,7 +98,9 @@ func (s *SCSITargetService) AddCommandQueue(tid int, scmd *api.SCSICommand) erro
 	lun := binary.LittleEndian.Uint64(scmd.Lun[:])
 	scmd.Device = target.Devices[lun]
 
-	log.Debugf("scsi opcode: 0x%x, LUN: %d", int(scmd.SCB[0]), binary.LittleEndian.Uint64(scmd.Lun[:]))
+	if log.GetLevel() >= log.DebugLevel {
+		log.Debugf("scsi opcode: 0x%x, LUN: %d", int(scmd.SCB[0]), lun)
+	}
 
 	if scmd.Device == nil {
 		scmd.Device = target.LUN0