fix regex, remove unsafe pointer usage, and add graceful HTTP shutdown

- Fix versionMatcher regex: gorilla/mux does not support (?:) syntax,
  and -dirty suffix was required instead of optional
- Replace unsafe.Pointer LUN casts with binary.LittleEndian.Uint64
  in sbc.go, scsi.go, and target.go
- Implement graceful HTTP server shutdown with 5s timeout using
  srv.Shutdown() instead of raw listener close
- Replace golang.org/x/net/context with standard library context
- Respect existing req.Cancel value in canceler to avoid overwriting
- Add early context cancellation check in Do() to fail fast

Based on review of PR #120 by @orzhang, with fixes applied.

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

pkg/api/client/transport/cancellable/canceler.go

@@ -14,7 +14,10 @@ import (
 )
 
 func canceler(client transport.Sender, req *http.Request) func() {
-	// TODO(djd): Respect any existing value of req.Cancel.
+	// Respect any existing value of req.Cancel.
+	if req.Cancel != nil {
+		return nop
+	}
 	ch := make(chan struct{})
 	req.Cancel = ch