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.

App/editor/InspectorPanel.cpp

@@ -13,7 +13,9 @@
 
 #include <imgui.h>
 
+#include <algorithm>
 #include <cstdio>
+#include <filesystem>
 #include <vector>
 #include <type_traits>
 
@@ -148,6 +150,8 @@ void InspectorPanel::Draw(const EditorContext& ctx, entt::entity selected)
                 ImGui::EndDisabled();
             }
         };
+        menuItem(static_cast<Engine::ECS::MeshRendererComponent*>(nullptr),
+                 "Mesh Renderer");
         menuItem(static_cast<Engine::ECS::LightComponent*>(nullptr), "Light");
         menuItem(static_cast<Engine::ECS::CameraComponent*>(nullptr), "Camera");
         menuItem(static_cast<Engine::ECS::AnimationComponent*>(nullptr), "Animation");
@@ -194,19 +198,79 @@ void InspectorPanel::Draw(const EditorContext& ctx, entt::entity selected)
             {
                 ImGui::BeginDisabled(true);   // 禁用 = 内容置灰（Unity 语义）
             }
-            if (const Engine::Model* model = ctx.assets->GetModel(mr->model);
-                model != nullptr)
+            const std::string cur = ctx.assets->Models().Path(mr->model);
+            // 模型资产槽（UE Details 资产槽语义）：下拉列 assets/ 下全部
+            // 模型文件，选中即 LoadModel 应用；已加载路径缓存去重复用
             {
-                const auto& clips = model->GetClips();
-                ImGui::Text("model: valid (%u clips, %u bones)",
-                            static_cast<unsigned>(clips.size()),
-                            static_cast<unsigned>(model->GetBoneCount()));
-                ImGui::TextDisabled("(mesh asset is read-only here)");
+                namespace fs = std::filesystem;
+                static std::vector<std::string> s_ModelFiles;
+                s_ModelFiles.clear();
+                if (fs::exists("assets"))
+                {
+                    for (const auto& entry :
+                         fs::recursive_directory_iterator("assets"))
+                    {
+                        if (!entry.is_regular_file())
+                        {
+                            continue;
+                        }
+                        std::string ext = entry.path().extension().string();
+                        std::transform(ext.begin(), ext.end(), ext.begin(),
+                                       [](unsigned char c)
+                                       { return std::tolower(c); });
+                        if (ext == ".gltf" || ext == ".glb" || ext == ".fbx" ||
+                            ext == ".obj")
+                        {
+                            s_ModelFiles.push_back(
+                                entry.path().generic_string());
+                        }
+                    }
+                    std::sort(s_ModelFiles.begin(), s_ModelFiles.end());
+                }
+                std::vector<const char*> opts;
+                opts.push_back("<none>");
+                for (const auto& f : s_ModelFiles)
+                {
+                    opts.push_back(f.c_str());
+                }
+                int sel = 0;
+                for (size_t k = 0; k < s_ModelFiles.size(); ++k)
+                {
+                    if (s_ModelFiles[k] == cur)
+                    {
+                        sel = static_cast<int>(k) + 1;
+                    }
+                }
+                ImGui::SetNextItemWidth(-1.0f);
+                if (ImGui::Combo("##model_slot", &sel, opts.data(),
+                                 static_cast<int>(opts.size())))
+                {
+                    mr->model = (sel <= 0)
+                        ? Engine::ModelRef{}
+                        : ctx.assets->LoadModel(s_ModelFiles[static_cast<size_t>(sel) - 1]);
+                }
+                ImGui::Spacing();
+            }
+            // 无效句柄不进 GetModel（缓存对无效/悬空句柄 FATAL，是 bug 信号）
+            if (mr->model.Valid())
+            {
+                if (const Engine::Model* model = ctx.assets->GetModel(mr->model);
+                    model != nullptr)
+                {
+                    const auto& clips = model->GetClips();
+                    ImGui::Text("model: valid (%u clips, %u bones)",
+                                static_cast<unsigned>(clips.size()),
+                                static_cast<unsigned>(model->GetBoneCount()));
+                }
+                else
+                {
+                    ImGui::TextColored(ImVec4(1.0f, 0.3f, 0.3f, 1.0f),
+                                       "model: INVALID (dangling ref)");
+                }
             }
             else
             {
-                ImGui::TextColored(ImVec4(1.0f, 0.3f, 0.3f, 1.0f),
-                                   "model: INVALID (dangling ref)");
+                ImGui::TextDisabled("no model assigned");
             }
             if (!mr->active)
             {
@@ -677,8 +741,10 @@ void InspectorPanel::Draw(const EditorContext& ctx, entt::entity selected)
                 ImGui::BeginDisabled(true);
             }
             const auto* mr = reg.try_get<Engine::ECS::MeshRendererComponent>(selected);
+            // 句柄无效不进 GetModel（缓存对无效句柄 FATAL；手选 <none> 即无效）
             const Engine::Model* model =
-                mr ? ctx.assets->GetModel(mr->model) : nullptr;
+                (mr && mr->model.Valid()) ? ctx.assets->GetModel(mr->model)
+                                          : nullptr;
 
             // 依赖提示：动画作用在 MeshRenderer 的模型骨骼上
             if (mr == nullptr)