Drop the unreachable single-record CultMesh variant refusal; tolerate duplicate catalog slots

A variant needs its base beside it, so a single-record store can never hold one:
that overload's refusal could not fire. SlotNames no longer throws on a catalog
that repeats a slot.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/GameCult.Caching/CultCache.cs

@@ -2919,7 +2919,7 @@ namespace GameCult.Caching
                     resolution.Descriptor,
                     new CultVariantDelta(record.Variant.BaseKey, record.Variant.Overrides)
                     {
-                        SlotNames = persisted?.Members.ToDictionary(member => member.Slot, member => member.MemberName)
+                        SlotNames = persisted?.Members.GroupBy(member => member.Slot).ToDictionary(group => group.Key, group => group.First().MemberName)
                     },
                     null);
             }

src/GameCult.Mesh/CultMeshSingleFileDocuments.cs

@@ -191,8 +191,6 @@ namespace GameCult.Mesh
                     $"CultCache document '{path}' has schema '{record.SchemaId}', expected '{expectedSchemaId}'.");
             }
 
-            RefuseVariant(path, record);
-
             if (!PublishesSchema(snapshot.SchemaCatalog, expectedSchemaId))
             {
                 throw new InvalidDataException(