nix: fix shallow-merge drop in module-eval's hive stub helper

nix/module-eval.nix

@@ -49,13 +49,20 @@ let
           };
           boot.loader.grub.enable = false;
           system.stateVersion = "25.11";
-          services.hyperhive = {
+          # `recursiveUpdate`, not `//`: a plain `//` only merges the
+          # *top-level* keys of `extra` in, so an `extra` that touches a
+          # nested attr under an existing top-level key (e.g. `swarm.*`)
+          # silently drops every sibling under that key instead of merging
+          # into it — the same class of bug as the `services.hyperhive`
+          # double-nesting mistake this stub already had to dodge once,
+          # just one level down. `recursiveUpdate` merges nested attrsets
+          # all the way down instead.
+          services.hyperhive = lib.recursiveUpdate {
             enable = true;
             hiveName = "h1";
             swarm.domain = "t.local";
             swarm.hives.h1.domain = "h1.t.local";
-          }
-          // extra;
+          } extra;
         }
       ];
     }).config;