M3: Hierarchy panel — click a GameObject to select it

HierarchyPanel walks IWorld.Roots/GameObject.Children recursively as
ImGui tree nodes, clicking one sets EditorState.Selected — the shared
selection the not-yet-built Inspector panel will read from the same
instance. PushID(go.GetHashCode()) scopes each node's ID by object
identity rather than name, since nothing stops two sibling GameObjects
sharing a Name and ImGui's default label-based IDs would otherwise merge
their open/selected state.

Also gives samples/WindowDemo/scene.json a child GameObject (offset,
half-scale, parented under Quad) — the existing scene only had one root,
nothing to show a tree with. Verified by screenshot: both quads render at
their correct composed WorldMatrix (the child visibly smaller and offset,
confirming parent/child composition is still correct through this
change), and Hierarchy lists "Quad" as a collapsible node.

Full suite still green: 65 tests.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N1qPfzq8TDCUMFMV3UwV5N

plugins/engine.editor/Engine.Editor/EditorPlugin.cs

@@ -19,14 +19,15 @@ namespace Engine.Editor;
 /// split exists at all.
 ///
 /// Nothing here reads or writes a Component through GetComponent/Query —
-/// the Hierarchy and Inspector panels (not built yet) will walk
-/// IWorld.Roots and GameObject.Components directly instead, which is why
-/// this plugin never needs to declare Reads/Writes on its system: those
-/// checks only guard the typed accessors, not plain property reads. See
-/// GameObject.AddComponent's own doc comment on the same gap.
+/// HierarchyPanel walks IWorld.Roots/GameObject.Children directly, and the
+/// not-yet-built Inspector will walk GameObject.Components the same way —
+/// which is why this plugin never needs to declare Reads/Writes on its
+/// system: those checks only guard the typed accessors, not plain property
+/// reads. See GameObject.AddComponent's own doc comment on the same gap.
 /// </summary>
 public sealed class EditorPlugin : IPlugin
 {
+    private readonly EditorState _state = new();
     private GL? _gl;
     private ImGuiController? _controller;
     private ITime? _time;
@@ -64,6 +65,8 @@ public sealed class EditorPlugin : IPlugin
         ImGui.Text($"Frame: {_time.FrameCount}");
         ImGui.End();
 
+        HierarchyPanel.Draw(world, _state);
+
         _controller.Render();
     }
 }

plugins/engine.editor/Engine.Editor/EditorState.cs

@@ -0,0 +1,15 @@
+using Engine.Kernel.World;
+
+namespace Engine.Editor;
+
+/// <summary>
+/// Shared, per-session editor state — currently just which GameObject the
+/// Hierarchy panel last clicked, so the Inspector panel (built against this
+/// same instance) knows what to show. One instance per EditorPlugin, not
+/// static: reloading engine.editor should start with nothing selected, not
+/// hold a reference to a GameObject that may not even exist anymore.
+/// </summary>
+internal sealed class EditorState
+{
+    public GameObject? Selected { get; set; }
+}

plugins/engine.editor/Engine.Editor/HierarchyPanel.cs

@@ -0,0 +1,50 @@
+using Engine.Kernel.World;
+using ImGuiNET;
+
+namespace Engine.Editor;
+
+/// <summary>
+/// Walks IWorld.Roots/GameObject.Children directly — not Query&lt;T&gt;(),
+/// there's no component type to query for here — so, like EditorPlugin
+/// itself, this never touches SystemAccessScope and needs no declared
+/// Reads/Writes.
+/// </summary>
+internal static class HierarchyPanel
+{
+    public static void Draw(IWorld world, EditorState state)
+    {
+        ImGui.Begin("Hierarchy");
+
+        foreach (var root in world.Roots)
+            DrawNode(root, state);
+
+        ImGui.End();
+    }
+
+    private static void DrawNode(GameObject go, EditorState state)
+    {
+        // PushID/PopID, not relying on go.Name for identity: sibling
+        // GameObjects can share a name (nothing stops it), and ImGui's
+        // default ID-from-label would then merge their open/selected state.
+        ImGui.PushID(go.GetHashCode());
+
+        var flags = ImGuiTreeNodeFlags.OpenOnArrow | ImGuiTreeNodeFlags.SpanAvailWidth;
+        if (go.Children.Count == 0)
+            flags |= ImGuiTreeNodeFlags.Leaf | ImGuiTreeNodeFlags.NoTreePushOnOpen;
+        if (ReferenceEquals(state.Selected, go))
+            flags |= ImGuiTreeNodeFlags.Selected;
+
+        var open = ImGui.TreeNodeEx(go.Name, flags);
+        if (ImGui.IsItemClicked())
+            state.Selected = go;
+
+        if (open && go.Children.Count > 0)
+        {
+            foreach (var child in go.Children)
+                DrawNode(child, state);
+            ImGui.TreePop();
+        }
+
+        ImGui.PopID();
+    }
+}