feat(editor): multi-script entities + asset browser panel

ScriptComponent now holds a list of {name, active} items (Unity
semantics: one entity, many scripts, each with its own enable bit;
component-level active stays as the master switch). ScriptRuntime
reconciles instances by name per entity -- rename/remove destroys
only the affected instances, untouched ones keep ticking. Scenes
persist Script.Items; old single-script key still loads.

AssetBrowserPanel (lightweight Unity Project window): fixed roots
(assets/scenes) as tabs, recursive directory tree, file list with
type filter (models/textures/scenes) and name substring filter,
double-click preloads models/HDRs into the asset caches (path-
deduped). Scene files point to the File menu for now; drag & drop
is a later iteration. Docked with Stats by default in fresh layouts.

App/editor/Editor.h

@@ -26,6 +26,7 @@ class HierarchyPanel;
 class InspectorPanel;
 class ViewportPanel;
 class StatsPanel;
+class AssetBrowserPanel;
 
 class Editor {
 public:
@@ -86,6 +87,7 @@ private:
     std::unique_ptr<InspectorPanel> m_Inspector;
     std::unique_ptr<ViewportPanel> m_Viewport;
     std::unique_ptr<StatsPanel> m_Stats;
+    std::unique_ptr<AssetBrowserPanel> m_AssetBrowser;
 
     entt::entity m_Selected{entt::null};
 
@@ -94,6 +96,7 @@ private:
     bool m_ShowInspector = true;
     bool m_ShowViewport = true;
     bool m_ShowStats = true;
+    bool m_ShowAssetBrowser = true;
 
     // 默认布局只建一次（imgui.ini 已有布局则跳过）
     bool m_LayoutBuilt = false;