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/backingstore/common.go

@@ -139,6 +139,14 @@ func (bs *FileBackingStore) Read(offset, tl int64) ([]byte, error) {
 	return tmpbuf, nil
 }
 
+// ReadAt reads directly into the provided buffer, avoiding allocation.
+func (bs *FileBackingStore) ReadAt(buf []byte, offset int64) (int, error) {
+	if bs.file == nil {
+		return 0, fmt.Errorf("Backend store is nil")
+	}
+	return bs.file.ReadAt(buf, offset)
+}
+
 func (bs *FileBackingStore) Write(wbuf []byte, offset int64) error {
 	length, err := bs.file.WriteAt(wbuf, offset)
 	if err != nil {