perf(client): lazy child detection in BrowseAsync, NodeId-indexed UI notifications

R08 3.6: BrowseAsync no longer issues one HasChildren Browse per Object child; an
Object reports HasChildren (may have children) and is confirmed when expanded or
recursed. ISessionAdapter.HasChildrenAsync is removed. The UI subscriptions panel
routes each notification through a NodeId index kept in step with
ActiveSubscriptions (add/remove/replace/reset) instead of scanning every row.

Refs #747
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT

tests/Client/ZB.MOM.WW.OtOpcUa.Client.Shared.Tests/OpcUaClientServiceTests.cs

@@ -384,10 +384,12 @@ public class OpcUaClientServiceTests : IDisposable
     }
 
     /// <summary>
-    /// Verifies that object nodes trigger child-detection checks so the client can mark expandable branches.
+    /// R08 3.6: browsing a level issues ONE Browse (plus BrowseNext pages) however many Object children it holds. Child
+    /// detection is lazy: an Object may have children and is confirmed when it is expanded/recursed, instead of one
+    /// extra Browse per Object child (~50 000 calls for a recursive subscribe over a 50k-node raw tree).
     /// </summary>
     [Fact]
-    public async Task BrowseAsync_ObjectNode_ChecksHasChildren()
+    public async Task BrowseAsync_ObjectChildren_MayHaveChildren_WithoutAPerChildBrowse()
     {
         var session = new FakeSessionAdapter
         {
@@ -398,17 +400,29 @@ public class OpcUaClientServiceTests : IDisposable
                     NodeId = new ExpandedNodeId("ns=2;s=Folder1"),
                     DisplayName = new LocalizedText("Folder1"),
                     NodeClass = NodeClass.Object
+                },
+                new ReferenceDescription
+                {
+                    NodeId = new ExpandedNodeId("ns=2;s=Folder2"),
+                    DisplayName = new LocalizedText("Folder2"),
+                    NodeClass = NodeClass.Object
+                },
+                new ReferenceDescription
+                {
+                    NodeId = new ExpandedNodeId("ns=2;s=Tag"),
+                    DisplayName = new LocalizedText("Tag"),
+                    NodeClass = NodeClass.Variable
                 }
-            ],
-            HasChildrenResponse = true
+            ]
         };
         _sessionFactory.EnqueueSession(session);
         await _service.ConnectAsync(ValidSettings());
 
         var results = await _service.BrowseAsync();
 
-        results[0].HasChildren.ShouldBeTrue();
-        session.HasChildrenCount.ShouldBe(1);
+        results.Select(r => r.HasChildren).ShouldBe([true, true, false]);
+        session.BrowseCount.ShouldBe(1);
+        session.BrowseNextCount.ShouldBe(0);
     }
 
     /// <summary>