feat(ui): add reconcile tick to self-heal lost worker broadcasts

TasksIslandViewModel now runs a periodic tick that diffs the flat Items
collection against SQLite and patches properties in place (capped at 500
rows, sharing the Phase 1 delta sequence guard so it never overtakes a
fresher broadcast). Never rebuilds rows or triggers Regroup/LoadForList, so
it stays decoupled from the parallel Phase 2b virtualization work. The same
cadence now refreshes the long-lived Worktrees Overview, Log Visualizer, and
Merge Helper selection overlays; short-lived modals are untouched.

src/ClaudeDo.Ui/ViewModels/Islands/TasksIslandViewModel.cs

@@ -1,5 +1,6 @@
 using System.Collections.ObjectModel;
 using System.Globalization;
+using Avalonia.Threading;
 using CommunityToolkit.Mvvm.ComponentModel;
 using CommunityToolkit.Mvvm.Input;
 using ClaudeDo.Data;
@@ -29,6 +30,8 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
     // one task can be in flight at once. Only the newest may write to the row.
     private readonly Dictionary<string, long> _deltaSeq = new();
     private long _deltaCounter;
+    // Phase 3 reconcile tick — see the block at the end of this class.
+    private readonly System.Timers.Timer _reconcileTimer = new(4_000);
 
     public event EventHandler? SelectionChanged;
     public event EventHandler? FocusAddTaskRequested;
@@ -127,11 +130,15 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
         }
         _langChangedHandler = (_, _) => RefreshLocalizedText();
         Loc.LanguageChanged += _langChangedHandler;
+        _reconcileTimer.Elapsed += (_, _) => Dispatcher.UIThread.Post(() => _ = ReconcileTickAsync());
+        _reconcileTimer.Start();
     }
 
     public void Dispose()
     {
         Loc.LanguageChanged -= _langChangedHandler;
+        _reconcileTimer.Stop();
+        _reconcileTimer.Dispose();
     }
 
     private void RefreshLocalizedText()
@@ -1225,4 +1232,81 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
         foreach (var i in Items) i.IsSelected = ReferenceEquals(i, value);
         SelectionChanged?.Invoke(this, EventArgs.Empty);
     }
+
+    // ── Phase 3: reconcile tick ──────────────────────────────────────────────
+    // Self-healing safety net for a lost broadcast: every few seconds, diff the flat `Items`
+    // master collection against SQLite and patch properties in place. Never rebuilds a row,
+    // never calls Regroup() (Phase 2b owns OverdueItems/OpenItems/CompletedItems/Regroup and is
+    // rewriting them in parallel), and never falls back to LoadForList — the delta path already
+    // owns that escalation.
+
+    // Above this many rows, only the first N (in Items order) are reconciled per tick, so the
+    // query cost stays bounded instead of growing with an ever-larger list.
+    internal const int ReconcileRowCap = 500;
+
+    // Set by the shell from the main window's WindowState; the tick no-ops while minimized.
+    public bool IsWindowVisible { get; set; } = true;
+
+    // Test seam: awaited right after the reconcile query returns and before its results are
+    // applied, so a test can land a fresher, higher-sequence update in that gap and exercise the
+    // stale-sequence guard deterministically. Always null outside tests.
+    internal Func<Task>? ReconcileTickTestBarrier { get; set; }
+
+    // Awaitable so tests can drive it deterministically (mirrors RefreshTaskFromWorkerAsync).
+    internal async Task ReconcileTickAsync(CancellationToken ct = default)
+    {
+        var list = _currentList;
+        if (!IsWindowVisible || list is null) return;
+
+        // Mirrors the full-reload guard in RefreshTaskFromWorkerAsync: matching virtual:queued /
+        // virtual:running depends on a Planning parent's children, not a single entity.
+        if (list.Kind == ListKind.Virtual &&
+            (list.Id == "virtual:queued" || list.Id == "virtual:running"))
+            return;
+
+        var ids = Items.Select(r => r.Id).Take(ReconcileRowCap).ToList();
+        if (ids.Count == 0) return;
+
+        // Same monotonic per-task sequence as the Phase 1 delta path, so whichever of the two
+        // started most recently for a given task id wins, regardless of completion order.
+        var seqByTaskId = new Dictionary<string, long>(ids.Count);
+        foreach (var id in ids)
+        {
+            var seq = ++_deltaCounter;
+            seqByTaskId[id] = seq;
+            _deltaSeq[id] = seq;
+        }
+
+        List<TaskEntity> entities;
+        try
+        {
+            var idSet = ids.ToHashSet();
+            await using var db = await _dbFactory.CreateDbContextAsync(ct);
+            entities = await db.Tasks
+                .Include(t => t.List)
+                .Include(t => t.Worktree)
+                .Where(t => idSet.Contains(t.Id))
+                .ToListAsync(ct);
+        }
+        catch (OperationCanceledException) { return; }
+        catch (Exception ex)
+        {
+            System.Diagnostics.Debug.WriteLine($"TasksIsland: reconcile tick failed ({ex.Message})");
+            return;
+        }
+
+        if (ReconcileTickTestBarrier is { } barrier) await barrier();
+
+        var byId = entities.ToDictionary(e => e.Id);
+        foreach (var id in ids)
+        {
+            // Superseded by a fresher delta refresh or a later tick that landed while this one
+            // was reading — its result is stale, discard it.
+            if (!_deltaSeq.TryGetValue(id, out var current) || current != seqByTaskId[id]) continue;
+            if (!byId.TryGetValue(id, out var entity)) continue; // deleted; the delta path removes rows, not the tick
+
+            var row = Items.FirstOrDefault(r => r.Id == id);
+            row?.UpdateFromEntity(entity);
+        }
+    }
 }