fix: enforce sentinel reactively in OnPlateAdded/OnPlateRemoved

Without this, RemoveEmptyPlates would destroy the sentinel with no
recovery, and tail-plate subscriptions would go stale after plate list
mutations. Added tests for both scenarios.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Core/PlateManager.cs

@@ -198,6 +198,9 @@ namespace OpenNest
 
         private void OnPlateAdded(object sender, ItemAddedEventArgs<Plate> e)
         {
+            if (!suppressNavigation && !batching)
+                EnsureSentinel();
+
             PlateListChanged?.Invoke(this, EventArgs.Empty);
 
             if (!suppressNavigation)
@@ -212,6 +215,9 @@ namespace OpenNest
             if (CurrentIndex >= Count && Count > 0)
                 CurrentIndex = Count - 1;
 
+            if (!suppressNavigation && !batching)
+                EnsureSentinel();
+
             PlateListChanged?.Invoke(this, EventArgs.Empty);
 
             if (!suppressNavigation)