Commit resync writes in one transaction

Each insert, delete and rescore update was its own commit to disk, so a
resync of about a thousand QSOs froze the window thread. LogStore has a
new Batch() scope that commits its writes together. ContestSession uses
it in AddRangeFromNetwork, ClearFromNetwork and Rescore.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/Nonemm.Session.Tests/FakeLogStore.cs

@@ -57,7 +57,16 @@ public sealed class FakeLogStore : LogStore
 
     public void Delete(string id) => qsos.RemoveAll(q => q.Id == id);
 
+    public IDisposable Batch() => new NoBatch();
+
     public void Dispose()
     {
     }
+
+    private sealed class NoBatch : IDisposable
+    {
+        public void Dispose()
+        {
+        }
+    }
 }