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();
     }
 }