Add redirect-chain rules per RFC 9110 §15.4

checker/collect.go

@@ -221,10 +221,16 @@ func runProbe(ctx context.Context, host, ip, scheme string, port uint16, timeout
 		// and a separate http.Client.Timeout would race with it.
 		CheckRedirect: func(req *http.Request, via []*http.Request) error {
 			prev := via[len(via)-1]
+			// req.Response is the 3xx response that triggered this hop;
+			// it carries the redirecting status code (301/302/307/308…).
+			status := 0
+			if req.Response != nil {
+				status = req.Response.StatusCode
+			}
 			redirectChain = append(redirectChain, RedirectStep{
 				From:   prev.URL.String(),
 				To:     req.URL.String(),
-				Status: 0, // populated post-hoc below if available
+				Status: status,
 			})
 			// The transport's DialContext is pinned to the original
 			// (ip, port) and TLS ServerName is pinned to the original