🐛 Fix: Block inherited Add method on AppItemGroup (#305)

* Fix: shadow inherited Add method to prevent silent state corruption in AppItemGroup

Shadows the inherited `Add` method with `new void Add(AppItem item)` that throws `NotSupportedException`. This ensures callers trying to directly `Add` to an `AppItemGroup` are immediately blocked, protecting the internal collection and filter state as validated by existing unit tests.

* Fix: document why shadowed Add method coexists with InsertItem guard

Adds explanatory documentation directly above the `public new void Add` method in `AppItemGroup` to clarify its purpose. The comment highlights that while `InsertItem` effectively prevents mutations via virtual dispatch, the shadowed `Add` method provides an immediate failure explicitly at the `AppItemGroup` API boundary to prevent developers from accidentally bypassing the tracking mechanism by calling the method directly on the class.

* Update Models/AppItemGroup.cs

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Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: Mike Knight <173126+mikekthx@users.noreply.github.com>

Author: google-labs-jules[bot]

Models/AppItemGroup.cs

@@ -41,6 +41,16 @@ public class AppItemGroup : BulkObservableCollection<AppItem>
     // Also true during base-class construction (field initializer runs before base ctor).
     private bool _suppressGuard = true;
 
+    // Shadows the non-virtual base Add to give callers an immediate, unambiguous
+    // failure. InsertItem also throws as defense-in-depth, but that requires going
+    // through the virtual dispatch chain; this makes the prohibition self-documenting
+    // on the public API. Internal ReplaceAllGuarded uses Collection<T>.Add →
+    // InsertItem (compile-time dispatch via BulkObservableCollection<T>) and never
+    // reaches this shadow.
+    public new void Add(AppItem item) =>
+        throw new NotSupportedException(
+            $"Direct Add/Insert bypasses _allItems and corrupts filter state. Use the {nameof(AppItemGroup)}-level APIs.");
+
     // Wraps ReplaceAll with the mutation guard suppressed, so ApplyFilter and ApplyCollapseState
     // can batch-replace the visible collection without the guard interfering.
     private void ReplaceAllGuarded(IEnumerable<AppItem> items)