fix: IBL 模型全黑——prefilter mip 拷贝偏移错位 + 采样器 maxLod 错误

- UploadCube 的 mip buffer 偏移误用当前 mip 尺寸×序号（mip 面递减，
  正确应为累积偏移）→ 各 mip 拷到 staging 错位数据，含未初始化 fp16
  NaN → 污染 ambient 输出整屏黑
- prefilter/irradiance 绑定改用环境采样器（maxLod=mipCount-1）：
  shadowSampler maxLod=0 会把 textureLod 显式 lod clamp 到 0，
  pre-filter 永远停在最锐 mip

Engine/src/Renderer/Cubemap.cpp

@@ -328,13 +328,17 @@ namespace Engine
             cmd.pipelineBarrier2(
                 vk::DependencyInfo{}.setImageMemoryBarriers(toTransfer));
 
+            // 逐 mip 逐面拷贝；buffer 偏移必须按 mips[0..mip) 字节数累积
+            //（mip 面尺寸递减，用当前 dim 乘 mip 序号会错位 → 读到
+            // staging 未初始化数据 → fp16 NaN 污染整屏）
+            vk::DeviceSize offset = 0;
             for (u32 mip = 0; mip < mipCount; ++mip) {
                 const u32 dim = mips[mip].faceSize;
                 for (u32 f = 0; f < kFaceCount; ++f) {
                     vk::BufferImageCopy region;
                     region.setBufferOffset(
-                               (static_cast<vk::DeviceSize>(dim) * dim * 8 *
-                                kFaceCount) * mip)
+                               offset + static_cast<vk::DeviceSize>(dim) *
+                                        dim * 8 * f)
                           .setBufferRowLength(0)
                           .setBufferImageHeight(0)
                           .setImageSubresource({vk::ImageAspectFlagBits::eColor,
@@ -345,6 +349,8 @@ namespace Engine
                                           vk::ImageLayout::eTransferDstOptimal,
                                           region);
                 }
+                offset += static_cast<vk::DeviceSize>(dim) * dim * 8 *
+                          kFaceCount;
             }
 
             vk::ImageMemoryBarrier2 toShader;