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/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; }
+}