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

samples/WindowDemo/scene.json

@@ -9,6 +9,19 @@
     "components": [
       { "type": "Engine.Render.Contracts.QuadRenderer, Engine.Render.Contracts", "data": {} }
     ],
-    "children": []
+    "children": [
+      {
+        "name": "ChildQuad",
+        "transform": {
+          "position": [1.5, 0, 0],
+          "rotation": [0, 0, 0, 1],
+          "scale": [0.5, 0.5, 1]
+        },
+        "components": [
+          { "type": "Engine.Render.Contracts.QuadRenderer, Engine.Render.Contracts", "data": {} }
+        ],
+        "children": []
+      }
+    ]
   }
 ]