feat(editor): manual mesh renderer setup + asset hover info card

Mesh Renderer was missing from the Add Component menu and its model
reference was read-only, so a mesh could only be attached by loading
a saved scene. Add it to the menu (Unity Add Component semantics) and
give the block a UE-Details-style asset slot: a dropdown listing all
model files under assets/, selecting applies LoadModel (cache deduped).

Asset browser files now show a UE Content Browser style hover card:
kind, size, modified time, and loaded state (new AssetCache::Contains
query, read-only -- hovering never triggers loading).

Fix latent FATALs unlocked by manual assignment: AssetCache::Get
aborts on invalid handles, so rendering, animation sampling, and mesh
collider build now guard mr.model.Valid() before calling it.

Engine/include/Engine/Assets/AssetManager.h

@@ -122,6 +122,12 @@ namespace Engine
                 return static_cast<u32>(m_Entries.size()) - static_cast<u32>(m_Free.size());
             }
 
+            // 路径是否在缓存（含被卸载后路径清除的语义：卸载即不含）
+            bool Contains(const std::string& path) const
+            {
+                return m_ByPath.find(path) != m_ByPath.end();
+            }
+
         private:
             struct Entry
             {
@@ -170,6 +176,16 @@ namespace Engine
                                    DecodedEquirect&& decoded);
             Cubemap* GetEnvironment(EnvRef h) const { return m_Environments.Get(h); }
 
+            // 路径缓存查询（面板显示加载状态用；不触发加载）
+            bool IsModelLoaded(const std::string& path) const
+            {
+                return m_Models.Contains(path);
+            }
+            bool IsEnvironmentLoaded(const std::string& path) const
+            {
+                return m_Environments.Contains(path);
+            }
+
             AssetCache<Model>& Models() { return m_Models; }
 
             // ---- 资产 GUID 注册表（UE AssetRegistry 模式：中央注册表）----