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.cpp

@@ -10,6 +10,7 @@
 //
 
 #include "Editor.h"
+#include "AssetBrowserPanel.h"
 #include "HierarchyPanel.h"
 #include "InspectorPanel.h"
 #include "ViewportPanel.h"
@@ -40,6 +41,7 @@ void Editor::Init(const EditorContext& ctx)
     m_Inspector = std::make_unique<InspectorPanel>();
     m_Viewport = std::make_unique<ViewportPanel>();
     m_Stats = std::make_unique<StatsPanel>();
+    m_AssetBrowser = std::make_unique<AssetBrowserPanel>();
 }
 
 void Editor::Draw()
@@ -82,6 +84,7 @@ void Editor::Draw()
         }
     }
     if (m_ShowStats)     m_Stats->Draw(m_Ctx);
+    if (m_ShowAssetBrowser) m_AssetBrowser->Draw(m_Ctx);
 
     DrawQuitModal();
     DrawSceneModals();
@@ -154,6 +157,7 @@ void Editor::DrawMenuBar()
         ImGui::MenuItem("Inspector", nullptr, &m_ShowInspector);
         ImGui::MenuItem("Viewport", nullptr, &m_ShowViewport);
         ImGui::MenuItem("Stats", nullptr, &m_ShowStats);
+        ImGui::MenuItem("Asset Browser", nullptr, &m_ShowAssetBrowser);
         ImGui::EndMenu();
     }
 
@@ -184,6 +188,7 @@ void Editor::DrawDockSpace()
 
         ImGui::DockBuilderDockWindow("Hierarchy", dockLeft);
         ImGui::DockBuilderDockWindow("Stats", dockStats);
+        ImGui::DockBuilderDockWindow("Asset Browser", dockStats);
         ImGui::DockBuilderDockWindow("Inspector", dockRight);
         ImGui::DockBuilderDockWindow("Viewport", dockCenter);