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/src/Physics/PhysicsWorld.cpp

@@ -685,8 +685,8 @@ namespace Engine {
         // 同实体 MeshRenderer 提供网格；缺失 → 回退（CreateBody 判 null 走 Box）
         auto *mc = m_Scene.GetRegistry().try_get<ECS::MeshRendererComponent>(
             static_cast<entt::entity>(entityBits));
-        if (m_Assets == nullptr || mc == nullptr) {
-            return nullptr;
+        if (m_Assets == nullptr || mc == nullptr || !mc->model.Valid()) {
+            return nullptr; // 无效句柄（未指派）不进 Get（对无效 FATAL）
         }
         const Model *model = m_Assets->GetModel(mc->model);
         if (model == nullptr) {