Diff IsMain in publication sync instead of special-casing it

DiffToUpdate now carries the false->true promotion like any other field (the CRDT side turns it into a SetMainPublicationChange), so the collection-level promotion special-case is removed. A main is never demoted, so there's no 1->0 handling to add.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: myieye

backend/FwLite/MiniLcm/SyncHelpers/PublicationSync.cs

@@ -9,26 +9,10 @@ public static async Task<int> Sync(Publication[] beforePublications,
         Publication[] afterPublications,
         IMiniLcmApi api)
     {
-        var changes = await DiffCollection.Diff(
+        return await DiffCollection.Diff(
             beforePublications,
             afterPublications,
             new PublicationsDiffApi(api));
-
-        // Sync the IsMain flag: if "after" has a main but "before" does not, promote it.
-        // A main that's new to this collection was already created with IsMain set (PublicationsDiffApi.Add),
-        // so only promote one that already existed but wasn't yet the main — otherwise we'd redundantly update
-        // a publication that doesn't exist yet (e.g. it throws during a dry-run sync).
-        var beforeMain = beforePublications.FirstOrDefault(p => p.IsMain);
-        var afterMain = afterPublications.FirstOrDefault(p => p.IsMain);
-
-        if (afterMain is not null && beforeMain is null && beforePublications.Any(p => p.Id == afterMain.Id))
-        {
-            await api.UpdatePublication(afterMain.Id,
-                new UpdateObjectInput<Publication>().Set(p => p.IsMain, true));
-            changes++;
-        }
-
-        return changes;
     }
 
     public static async Task<int> Sync(
@@ -51,7 +35,9 @@ public static async Task<int> Sync(
             beforePublication.Name,
             afterPublication.Name));
 
-        // IsMain is intentionally not diffed here: it transitions only 0->1, handled once in the collection-level Sync.
+        if (beforePublication.IsMain != afterPublication.IsMain)
+            patchDocument.Replace(p => p.IsMain, afterPublication.IsMain);
+
         if (patchDocument.Operations.Count == 0) return null;
         return new UpdateObjectInput<Publication>(patchDocument);
     }