fix(central-ui): InstanceTreeBuilder — orphaned instances surface at root; read-only Children; edge tests

Review follow-ups on f12ba2f9:
- An instance whose AreaId points at an area outside the supplied list was
  silently dropped; it now surfaces at the root, mirroring the orphaned-area
  rule, and stays filterable.
- InstanceTreeNode.Children is an IReadOnlyList over a private backing list,
  composed by the builder through an internal AddChild — the record no longer
  hands callers a mutable list.
- Extra tests: unknown-area instance (root + filterable), 3-level nesting,
  a ParentAreaId cycle that must neither loop nor throw, and a filter that
  matches instance names only (an area name match alone does not keep it).

Claude-Session: https://claude.ai/code/session_01NcMG6xXdsJbmD2wYAgkK29

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Shared/InstanceTreeBuilderTests.cs

@@ -82,6 +82,84 @@ public class InstanceTreeBuilderTests
         Assert.Equal("i:10", leaf.Key);
     }
 
+    [Fact]
+    public void Build_InstanceWithUnknownArea_SurfacesAtRoot()
+    {
+        // AreaId points at an area that is not in the supplied list — the instance must not
+        // vanish; it is listed at the root like an instance with no area at all.
+        var line1 = MakeArea(1, "Line1");
+        var stranded = MakeInstance(10, "Stranded", areaId: 99);
+
+        var roots = InstanceTreeBuilder.Build([line1], [stranded], filter: null);
+
+        Assert.Equal(new[] { "a:1", "i:10" }, roots.Select(n => n.Key));
+        Assert.Same(stranded, roots[1].Instance);
+    }
+
+    [Fact]
+    public void Build_InstanceWithUnknownArea_StaysFilterable()
+    {
+        var line1 = MakeArea(1, "Line1");
+        var stranded = MakeInstance(10, "Stranded", areaId: 99);
+
+        var matched = InstanceTreeBuilder.Build([line1], [stranded], filter: "rand");
+
+        Assert.Equal("i:10", Assert.Single(matched).Key);
+        Assert.Empty(InstanceTreeBuilder.Build([line1], [stranded], filter: "nomatch"));
+    }
+
+    [Fact]
+    public void Build_DeepNesting_WiresChildrenAtEveryLevel()
+    {
+        var plant = MakeArea(1, "Plant");
+        var line = MakeArea(2, "Line", parentAreaId: 1);
+        var cell = MakeArea(3, "Cell", parentAreaId: 2);
+        var deep = MakeInstance(10, "Deep", areaId: 3);
+
+        var roots = InstanceTreeBuilder.Build([plant, line, cell], [deep], filter: null);
+
+        var plantNode = Assert.Single(roots);
+        Assert.Equal("a:1", plantNode.Key);
+        var lineNode = Assert.Single(plantNode.Children);
+        Assert.Equal("a:2", lineNode.Key);
+        var cellNode = Assert.Single(lineNode.Children);
+        Assert.Equal("a:3", cellNode.Key);
+        var leaf = Assert.Single(cellNode.Children);
+        Assert.Equal("i:10", leaf.Key);
+        Assert.False(leaf.HasChildren);
+    }
+
+    [Fact]
+    public void Build_CyclicParentAreaIds_DoNotLoopOrThrow()
+    {
+        // A bad row pair: each area names the other as its parent. Neither is reachable
+        // from a root, so the forest is simply empty — and building it must terminate.
+        var a = MakeArea(1, "A", parentAreaId: 2);
+        var b = MakeArea(2, "B", parentAreaId: 1);
+        var inA = MakeInstance(10, "InA", areaId: 1);
+        var inB = MakeInstance(11, "InB", areaId: 2);
+
+        var roots = InstanceTreeBuilder.Build([a, b], [inA, inB], filter: null);
+
+        Assert.Empty(roots);
+    }
+
+    [Fact]
+    public void Build_Filter_MatchesInstanceNamesOnly_AreaNameMatchIsNotEnough()
+    {
+        // "Line1" contains the needle but holds no matching instance — it is dropped.
+        var line1 = MakeArea(1, "Line1");
+        var line2 = MakeArea(2, "Line2");
+        var other = MakeInstance(10, "Pump", areaId: 1);
+        var hit = MakeInstance(11, "Line1Feeder", areaId: 2);
+
+        var roots = InstanceTreeBuilder.Build([line1, line2], [other, hit], filter: "line1");
+
+        var root = Assert.Single(roots);
+        Assert.Equal("a:2", root.Key);
+        Assert.Equal("i:11", Assert.Single(root.Children).Key);
+    }
+
     [Fact]
     public void Build_Filter_KeepsMatchingInstancesWithAncestors_DropsEmptyBranches()
     {