refactor(registry): compose stable uid validation

registry/stable-uid.nix

@@ -1,28 +1,19 @@
 { lib, ... }:
 let
   inherit (lib)
-    foldl'
+    filterAttrs
     groupBy
+    mapAttrs
     unique
     ;
 
+  serviceNamesByUid = entries:
+    mapAttrs (_uid: uidEntries: unique (map (entry: entry.entry) uidEntries)) (
+      groupBy (entry: toString entry.value) entries
+    );
+
   duplicateUids = entries:
-    let
-      byUid = groupBy (entry: toString entry.value) entries;
-    in
-    foldl' (
-      acc: uid:
-      let
-        serviceNames = unique (map (entry: entry.entry) byUid.${uid});
-      in
-      if builtins.length serviceNames > 1 then
-        acc ++ [ {
-          uid = lib.toInt uid;
-          inherit serviceNames;
-        } ]
-      else
-        acc
-    ) [ ] (builtins.attrNames byUid);
+    filterAttrs (_uid: serviceNames: builtins.length serviceNames > 1) (serviceNamesByUid entries);
 
 in
 {
@@ -37,7 +28,7 @@ in
     in
     [
       {
-        assertion = uidConflicts == [ ];
+        assertion = uidConflicts == { };
         message = "Stable UID allocations must be unique.";
       }
     ];