fix(gemini): reject resume when caller supplies Writer

internal/protocol/gemini/gemini.go

@@ -207,6 +207,16 @@ func (p *Protocol) downloadWithRedirects(ctx context.Context, req *core.Download
 	// fresh downloads (create) and resume (append). When req.Writer is
 	// set (caller manages the file), we use it as-is.
 	if req.Output != "" && req.Output != "-" {
+		// Resume is meaningful only when we own the output file. If the
+		// caller already supplied a Writer we cannot append to Output
+		// behind their back, so disable resume to keep the partial-state
+		// sidecar consistent with what actually lands on disk.
+		if req.Resume && req.Writer != nil && req.Verbose {
+			fmt.Fprintf(os.Stderr, "[gemini] warning: --resume ignored because a Writer is set; output goes to the Writer only\n")
+		}
+		if req.Resume && req.Writer != nil {
+			req.Resume = false
+		}
 		if req.Resume {
 			canResume, resumeMeta, rerr := output.CanResume(req.Output, req.URL.String())
 			if rerr != nil {