fix(ui): stop the reconcile ticks from destroying user state

The 4s reconcile tick was added to three long-lived surfaces. On two of
them it reloads by rebuilding every row instance, which silently threw
away whatever the user had built up since the overlay opened; on the
third it healed a row's data but left it in the wrong section.

- WorktreesOverview: LoadAsync clears Rows, SelectedCount, ConflictRows
  and BatchProgress, so batch-merge ticks, the highlighted row and a
  finished batch's outcome badges were wiped every four seconds --
  assembling a multi-worktree selection was impossible. Carry that state
  across the reload, keyed by task id, and re-point SelectedRow at the
  fresh instance (or clear it when the worktree is gone).
- MergeHelperSelection: the remember/restore had no re-entrancy guard, so
  a second tick entering between the first one's reload and its restore
  snapshotted rows already back at IsTickedByDefault and wrote that
  default back, re-ticking what the user had unticked. One tick at a time,
  and hold the row instances instead of a value snapshot so a tick landed
  during the await survives.
- TasksIsland: the tick deliberately never called Regroup because Phase 2b
  owned Rows in parallel. 2b has landed, so a healed task that went Done
  stayed in the Open section under a stale count, and a healed depends-on
  link never pulled its dependent under the chain head. Regroup when a
  patch moved a grouping input, gated on a cheap key so an idle tick stays
  free.

src/ClaudeDo.Ui/ViewModels/Modals/MergeHelperSelectionModalViewModel.cs

@@ -45,6 +45,16 @@ public sealed partial class MergeHelperSelectionModalViewModel : ViewModelBase
     // doesn't cascade over its children's own remembered state.
     private bool _suppressCascade;
 
+    // Only one tick may be in flight. A second one entering while the first awaits LoadAsync
+    // would snapshot an already-cleared Tasks collection, restore that emptiness, and let
+    // IsTickedByDefault re-tick every row — silently undoing the user's unticks, i.e. exactly
+    // what the remember/restore below exists to prevent.
+    private bool _tickInFlight;
+
+    // Test seam: awaited inside ReconcileTickAsync after the reload and before the restore, so a
+    // test can hold one tick open in exactly the window a second one would corrupt. Null in prod.
+    internal Func<Task>? ReconcileTickTestBarrier { get; set; }
+
     public ObservableCollection<MergeHelperTaskRowViewModel> Tasks { get; } = new();
 
     [ObservableProperty] private string _scopeLabel = "";
@@ -81,25 +91,43 @@ public sealed partial class MergeHelperSelectionModalViewModel : ViewModelBase
     // already existed across the reload; only genuinely new rows get the default.
     internal async Task ReconcileTickAsync()
     {
-        if (string.IsNullOrEmpty(_listId)) return;
-        var previous = Tasks.ToDictionary(t => t.Id, t => t.IsSelected);
-        await LoadAsync();
-
-        // Restore without the parent→children cascade: it would overwrite a child's own remembered
-        // state with its parent's, undoing exactly the per-child choice being restored.
-        _suppressCascade = true;
+        if (string.IsNullOrEmpty(_listId) || _tickInFlight) return;
+        _tickInFlight = true;
         try
         {
-            foreach (var t in Tasks)
-                if (previous.TryGetValue(t.Id, out var wasSelected))
-                    t.IsSelected = wasSelected;
+            // Hold the row INSTANCES, not a value snapshot, and read their state back only after
+            // the reload: a tick the user lands during the await happens before LoadAsync clears
+            // the collection, so the live instance still carries it. Snapshotting up front would
+            // drop that click.
+            var previousRows = Tasks.ToList();
+            await LoadAsync();
+
+            if (ReconcileTickTestBarrier is { } barrier) await barrier();
+
+            var previous = new Dictionary<string, bool>(previousRows.Count);
+            foreach (var t in previousRows) previous[t.Id] = t.IsSelected;
+
+            // Restore without the parent→children cascade: it would overwrite a child's own
+            // remembered state with its parent's, undoing exactly the per-child choice being
+            // restored.
+            _suppressCascade = true;
+            try
+            {
+                foreach (var t in Tasks)
+                    if (previous.TryGetValue(t.Id, out var wasSelected))
+                        t.IsSelected = wasSelected;
+            }
+            finally
+            {
+                _suppressCascade = false;
+            }
+
+            OnPropertyChanged(nameof(CanConfirm));
         }
         finally
         {
-            _suppressCascade = false;
+            _tickInFlight = false;
         }
-
-        OnPropertyChanged(nameof(CanConfirm));
     }
 
     public async Task LoadAsync(CancellationToken ct = default)