fix(navigationview): consume reuse entries + explicit Where (CR feedback)

Address PR #454 review:
- Slow-path rebuild now consumes the reuse map entry (Dictionary.Remove)
  when a container is reused, so duplicate sibling keys (duplicate Tags, or
  duplicate Content when no Tag is set) fall through to a freshly created
  container instead of adding the same WinUI item to the collection twice.
- Make the snapshot loops' filtering explicit with .Where()/.OfType().

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: codemonkeychris

src/Reactor/Core/Reconciler.Update.cs

@@ -1873,15 +1873,13 @@ private void ReconcileNavMenuItems(
         // existing containers by Tag so matches can be reused, then rebuild the
         // collection in the new order. Reused containers retain their IsExpanded.
         var reusable = new global::System.Collections.Generic.Dictionary<string, WinUI.NavigationViewItem>();
-        foreach (var item in live)
-            if (item is WinUI.NavigationViewItem nvi && nvi.Tag is string tag)
-                reusable[tag] = nvi;
+        foreach (var nvi in live.OfType<WinUI.NavigationViewItem>().Where(x => x.Tag is string))
+            reusable[(string)nvi.Tag] = nvi;
 
         var oldByTag = new global::System.Collections.Generic.Dictionary<string, NavigationViewItemData>();
         if (oldData is not null)
-            foreach (var d in oldData)
-                if (!d.IsHeader)
-                    oldByTag[d.Tag ?? d.Content] = d;
+            foreach (var d in oldData.Where(d => !d.IsHeader))
+                oldByTag[d.Tag ?? d.Content] = d;
 
         live.Clear();
         foreach (var data in newData)
@@ -1892,8 +1890,11 @@ private void ReconcileNavMenuItems(
                 continue;
             }
 
+            // Consume the reuse entry so duplicate sibling keys (duplicate Tags,
+            // or duplicate Content when no Tag is set) fall through to a fresh
+            // container rather than adding the same WinUI item to live twice.
             var key = data.Tag ?? data.Content;
-            if (reusable.TryGetValue(key, out var nvi))
+            if (reusable.Remove(key, out var nvi))
                 UpdateNavItemInPlace(nvi, oldByTag.GetValueOrDefault(key), data);
             else
                 nvi = CreateNavItem(data);

src/Reactor/Core/V1Protocol/Descriptor/Descriptors/NavigationViewDescriptor.cs

@@ -175,15 +175,13 @@ private static void ReconcileMenuItems(
         }
 
         var reusable = new global::System.Collections.Generic.Dictionary<string, WinUI.NavigationViewItem>();
-        foreach (var item in live)
-            if (item is WinUI.NavigationViewItem nvi && nvi.Tag is string tag)
-                reusable[tag] = nvi;
+        foreach (var nvi in live.OfType<WinUI.NavigationViewItem>().Where(x => x.Tag is string))
+            reusable[(string)nvi.Tag] = nvi;
 
         var oldByTag = new global::System.Collections.Generic.Dictionary<string, NavigationViewItemData>();
         if (oldData is not null)
-            foreach (var d in oldData)
-                if (!d.IsHeader)
-                    oldByTag[d.Tag ?? d.Content] = d;
+            foreach (var d in oldData.Where(d => !d.IsHeader))
+                oldByTag[d.Tag ?? d.Content] = d;
 
         live.Clear();
         foreach (var data in newData)
@@ -194,8 +192,10 @@ private static void ReconcileMenuItems(
                 continue;
             }
 
+            // Consume the reuse entry so duplicate sibling keys fall through to a
+            // fresh container rather than adding the same WinUI item to live twice.
             var key = data.Tag ?? data.Content;
-            if (reusable.TryGetValue(key, out var nvi))
+            if (reusable.Remove(key, out var nvi))
                 UpdateNavItemInPlace(nvi, oldByTag.GetValueOrDefault(key), data);
             else
                 nvi = CreateNavItem(data);