feat: 显存分配器迁移到 VMA（P1 路线第一项）

- vendored VulkanMemoryAllocator 子模块，唯一实现编译单元 VmaImpl.cpp
- MemoryAllocator 重写为 VMA 后端：接口从"Allocate(reqs)+手绑"升级为
  "创建即分配即绑定"（CreateBuffer/CreateImage/Destroy/Flush），
  VMA 要求资源与内存成对释放，约束成单一出口
- Buffer/Texture/RenderGraph 三处调用方迁移；staging 持久映射 + 按分配
  精确 flush（原来按整块 flush）
- 获得：TLSF 通用分配、DedicatedAllocation 自动启用、memory budget 感知、
  内存别名基础（TAA/复用前置）
- 编译零错误、validation 零警告、8 秒冒烟通过

Engine/src/RenderGraph/RenderGraph.cpp

@@ -45,8 +45,7 @@ namespace Engine::RG {
             for (auto& img : images) {
                 if (!img->valid) continue;
                 vkDevice.destroyImageView(img->view);
-                vkDevice.destroyImage(img->image);
-                m_Allocator.Free(img->allocation);
+                m_Allocator.Destroy(img->allocation);   // image 资源与显存成对释放
             }
         }
         m_Pool.clear();
@@ -530,8 +529,7 @@ namespace Engine::RG {
                 for (auto& img : it->second) {
                     if (!img->valid) continue;
                     vkDevice.destroyImageView(img->view);
-                    vkDevice.destroyImage(img->image);
-                    m_Allocator.Free(img->allocation);
+                    m_Allocator.Destroy(img->allocation);
                 }
                 it = m_Pool.erase(it);
             } else {
@@ -576,11 +574,10 @@ namespace Engine::RG {
                  .setSharingMode(vk::SharingMode::eExclusive)
                  .setInitialLayout(vk::ImageLayout::eUndefined);
 
-        img->image = vkDevice.createImage(imageInfo);
-
-        auto reqs = vkDevice.getImageMemoryRequirements(img->image);
-        img->allocation = m_Allocator.Allocate(reqs, vk::MemoryPropertyFlagBits::eDeviceLocal);
-        vkDevice.bindImageMemory(img->image, img->allocation.Memory, img->allocation.Offset);
+        // VMA：创建 + 分配 + 绑定一步完成
+        img->allocation = m_Allocator.CreateImage(imageInfo,
+            vk::MemoryPropertyFlagBits::eDeviceLocal);
+        img->image = img->allocation.Image;
 
         vk::ImageViewCreateInfo viewInfo;
         viewInfo.setImage(img->image)