feat(render): implement Campaign AR and terrain fidelity

src/AcDream.App/UI/UiTemplateListBox.cs

@@ -272,6 +272,29 @@ public sealed class UiTemplateListBox : UiDatElement
         return row;
     }
 
+    /// <summary>
+    /// Removes the dynamically-owned suffix after <paramref name="retainedItemCount"/>
+    /// without disturbing the retained prefix. This is the structural seam used by
+    /// optional Config-page extensions whose schema can change at runtime: retail's
+    /// authored prefix remains mounted, while only extension rows are detached.
+    /// Existing scroll is preserved and clamped by <see cref="UiScrollablePanel"/>.
+    /// </summary>
+    public void RemoveTail(int retainedItemCount)
+    {
+        int count = _viewport?.Children.Count ?? 0;
+        if (retainedItemCount < 0 || retainedItemCount > count)
+            throw new ArgumentOutOfRangeException(nameof(retainedItemCount));
+        if (_viewport is null || retainedItemCount == count)
+            return;
+
+        for (int i = count - 1; i >= retainedItemCount; i--)
+            _viewport.RemoveChild(_viewport.Children[i]);
+    }
+
+    /// <summary>Current number of materialized row widgets. Reading this does
+    /// not wake a dormant list box.</summary>
+    public int ItemCount => _viewport?.Children.Count ?? 0;
+
     /// <summary>
     /// Campaign FA slice FA3: removes every row previously added via
     /// <see cref="AddItemFromTemplateList"/>/<see cref="AddPrebuiltRow"/>, resetting