feat(adapter): add passthrough adapter for unchanged vectors

cmd/vecna/convert.go

@@ -95,6 +95,9 @@ func openWriter(path string) (io.Writer, error) {
 // buildAdapter constructs the Adapter from the loaded config.
 func buildAdapter(cfg *config.Config) (adapter.Adapter, error) {
 	ac := cfg.Adapter
+	if ac.SourceDim == 0 && ac.TargetDim == 0 {
+		return adapter.NewPassthrough(), nil
+	}
 	switch ac.Type {
 	case "truncate":
 		tm, pm, err := parseTruncateModes(ac.TruncateMode, ac.PadMode)