Fix selection outline never showing for the first entity

entt entity 0's integral bits are 0, colliding with the "no selection"
sentinel: u32 selected == 0 meant "clear selection", so the scene's
first-created entity could never be outlined. Replaced with
std::optional<u32> + FrameSceneData::selectedEntityValid.

Also restore per-batch instance contiguity broken by the batchMats
flattening (e038cb3): entities sharing a mesh scatter across the flat
instance array in collection order. ViewScratch now tracks per-batch
singly-linked lists and appendView aggregates each batch into a
contiguous range, preserving the flat-array performance intent.

Hierarchy click selection now routes through SelectEntity so outline
stays in sync when picking from the panel.

App/editor/Editor.cpp

@@ -131,7 +131,16 @@ void Editor::Draw()
     ImGui::End();
 
     // 常规面板窗口（窗口名 = docking 身份）
-    if (m_ShowHierarchy) m_Hierarchy->Draw(m_Ctx, m_Selected);
+    if (m_ShowHierarchy)
+    {
+        const entt::entity prev = m_Selected;
+        m_Hierarchy->Draw(m_Ctx, m_Selected);
+        // 树点选同步描边（面板直写 m_Selected，不走 Viewport 的
+        // SelectEntity 返回值通道；对比前后变化触发）
+        if (m_Selected != prev) {
+            SelectEntity(m_Selected);
+        }
+    }
     if (m_ShowInspector) m_Inspector->Draw(m_Ctx, m_Selected);
     if (m_ShowViewport)
     {