feat: 解析式 PBR（Cook-Torrance GGX，metallic-roughness 工作流）

- 修 void* 指针算术警告（Texture::InitFromDds）
- Material：slot2 语义切为 metallicRoughness（glTF 约定 G=粗糙度
  B=金属度）+ 新增材质因子参数 UBO（binding 4，per-slot ring，
  创建时写死，metallic/roughness 因子乘 MR 贴图）
- simple_shader.frag：Blinn-Phong → Cook-Torrance（GGX 分布 +
  Smith 几何 + Schlick Fresnel，k=(r+1)²/8 解析光源版）；金属不参与
  漫反射；环境光暂为常数（IBL 后续）
- Model：读 aiTextureType_UNKNOWN（glTF MR 落点）+ AI_MATKEY_
  METALLIC/ROUGHNESS_FACTOR（旧资产缺键退化为电介质 0/0.5）；
  材质缓存键含因子
- main.cpp：演示场景换回 backpack（旧资产走因子路径验证 PBR 全链路）
- 旧 specular 贴图不再使用（语义被 MR 取代）

Engine/src/Renderer/Model.cpp

@@ -38,13 +38,13 @@ Model::~Model() = default;
 void Model::InitDefaultTextures()
 {
     // 各材质槽位的兜底纹理（1x1，开销可忽略）：
-    //   albedo 白（受光） / normal 平法线 (0.5,0.5,1) / specular 黑 / emissive 黑
+    //   albedo 白 / MR 平参数（roughness 0.5, metallic 0）/ emissive 黑
     m_DefaultTextures[Material::Albedo] =
         Texture::CreateSolidColor(m_Device, m_Allocator, 255, 255, 255);
+    m_DefaultTextures[Material::MetallicRoughness] =
+        Texture::CreateSolidColor(m_Device, m_Allocator, 0, 128, 0, false);
     m_DefaultTextures[Material::Normal] =
         Texture::CreateSolidColor(m_Device, m_Allocator, 128, 128, 255, false);
-    m_DefaultTextures[Material::Specular] =
-        Texture::CreateSolidColor(m_Device, m_Allocator, 0, 0, 0, false);
     m_DefaultTextures[Material::Emissive] =
         Texture::CreateSolidColor(m_Device, m_Allocator, 0, 0, 0);
 }
@@ -74,7 +74,7 @@ Model::Model(Device& device, MemoryAllocator& allocator,
 
     // 材质/纹理生命周期随模型（缓存持有）
     std::string paths[Material::kSlotCount] = {"", "", "", ""};
-    Material* mat = GetOrCreateMaterial(paths);   // 全默认材质
+    Material* mat = GetOrCreateMaterial(paths, MaterialParams{});   // 全默认材质
 
     m_Meshes.push_back(CreateScope<Mesh>(
         m_Device, m_Allocator, vertices, indices, mat));
@@ -148,9 +148,10 @@ Texture* Model::GetOrLoadTexture(const std::string& rawPath, bool srgb)
     return ptr;
 }
 
-Material* Model::GetOrCreateMaterial(const std::string paths[Material::kSlotCount])
+Material* Model::GetOrCreateMaterial(const std::string paths[Material::kSlotCount],
+                                     const MaterialParams& params)
 {
-    const std::string key = Material::MakeCacheKey(paths);
+    const std::string key = Material::MakeCacheKey(paths, params);
     if (auto it = m_MaterialCache.find(key); it != m_MaterialCache.end()) {
         return it->second.get();
     }
@@ -169,7 +170,8 @@ Material* Model::GetOrCreateMaterial(const std::string paths[Material::kSlotCoun
     }
 
     auto mat = CreateScope<Material>(
-        m_Device, m_DescriptorManager, m_TextureLayout, textures);
+        m_Device, m_Allocator, m_DescriptorManager, m_TextureLayout,
+        textures, params);
     Material* ptr = mat.get();
     m_MaterialCache.emplace(key, std::move(mat));
     return ptr;
@@ -225,18 +227,20 @@ Scope<Mesh> Model::ProcessMesh(aiMesh* mesh, const aiScene* scene)
         }
     }
 
-    // 读取材质纹理路径（4 槽：albedo/normal/specular/emissive）
+    // 读取材质纹理路径（PBR metallic-roughness 槽位）+ 金属度/粗糙度因子
     std::string texPaths[Material::kSlotCount];
+    MaterialParams params;
 
     if (mesh->mMaterialIndex >= 0) {
         aiMaterial* material = scene->mMaterials[mesh->mMaterialIndex];
         const std::filesystem::path modelDir =
             std::filesystem::path(m_ModelPath).parent_path();
 
+        // glTF 的 metallicRoughness 贴图落在 aiTextureType_UNKNOWN
         const std::pair<aiTextureType, u32> slots[] = {
             {aiTextureType_DIFFUSE,  Material::Albedo},
             {aiTextureType_NORMALS,  Material::Normal},
-            {aiTextureType_SPECULAR, Material::Specular},
+            {aiTextureType_UNKNOWN,  Material::MetallicRoughness},
             {aiTextureType_EMISSIVE, Material::Emissive},
         };
 
@@ -247,10 +251,17 @@ Scope<Mesh> Model::ProcessMesh(aiMesh* mesh, const aiScene* scene)
                 texPaths[slot] = (modelDir / path.C_Str()).string();
             }
         }
+
+        // PBR 因子（glTF2 材质属性；旧资产无此键 → 默认电介质 0/0.5）
+        ai_real metallic = 0.0f, roughness = 0.5f;
+        material->Get(AI_MATKEY_METALLIC_FACTOR, metallic);
+        material->Get(AI_MATKEY_ROUGHNESS_FACTOR, roughness);
+        params.factors = glm::vec4(static_cast<f32>(metallic),
+                                   static_cast<f32>(roughness), 0.0f, 0.0f);
     }
 
-    // 材质缓存：同贴图组合 → 同一 Material（一个描述符集），mesh 数 ≠ 集数
-    Material* mat = GetOrCreateMaterial(texPaths);
+    // 材质缓存：同贴图组合+因子 → 同一 Material（一个描述符集）
+    Material* mat = GetOrCreateMaterial(texPaths, params);
 
     return CreateScope<Mesh>(
         m_Device, m_Allocator, vertices, indices, mat);