feat: 环境背景管线（IBL 阶段①）——HDR 等距柱状转 cube + Skybox pass

- Cubemap：stbi_loadf 加载 .hdr，CPU 双线性重采样 6 面（GL 规范方向表），
  f32→fp16 转换后整块上传 eCubeCompatible image（R16G16B16A16Sfloat）
- SkyboxPass：HDRColor 的 first writer，承担清屏职责；无环境贴图自动
  退化为纯清屏（Forward 不受影响）
- frag 逐像素 NDC 反投影取世界方向（invVP push，投影变换不可线性插值）
- Forward color 附件 Clear→Load：几何合成在天空之上
- SceneRenderer 产出 skyInvVP = inverse(proj × mat4(mat3(view)))

Engine/src/Renderer/Renderer.cpp

@@ -12,8 +12,10 @@
 #include <Engine/Renderer/Passes/SceneGpu.h>
 #include <Engine/Renderer/Passes/FullscreenGpu.h>
 #include <Engine/Renderer/Passes/RenderPass.h>
+#include <Engine/Renderer/Passes/SkyboxPass.h>
 #include <Engine/Renderer/Passes/ShadowPass.h>
 #include <Engine/Renderer/Passes/ForwardPass.h>
+#include <Engine/Renderer/Cubemap.h>
 #include <Engine/Renderer/Passes/TAAPass.h>
 #include <Engine/Renderer/Passes/BloomPass.h>
 #include <Engine/Renderer/Passes/TonemapPass.h>
@@ -38,6 +40,7 @@
 #include <glm/glm.hpp>
 
 #include <array>
+#include <filesystem>
 #include <limits>
 
 namespace Engine
@@ -71,6 +74,14 @@ namespace Engine
         m_FullscreenGpu = CreateScope<Passes::FullscreenGpu>(
             *m_Device, *m_DescriptorManager, *m_ShaderManager);
 
+        // 3.5 环境贴图（IBL 阶段①）：HDR 等距柱状 → cube。
+        //     文件缺失则跳过（SkyboxPass 自动退化为纯清屏，Forward 不受影响）
+        const std::string envPath = "assets/textures/venice_dawn_1_4k.hdr";
+        if (std::filesystem::exists(envPath)) {
+            m_Environment = Cubemap::CreateFromHdrEquirect(
+                *m_Device, *m_Allocator, envPath, 1024);
+        }
+
         // 4. Swapchain（只管呈现；渲染目标统一归 RenderGraph）
         m_Swapchain = CreateScope<Swapchain>(*m_Device, *m_Allocator, m_Window);
         m_Extent = m_Swapchain->GetExtent();
@@ -228,6 +239,10 @@ namespace Engine
         FrameSceneData& frame = m_SceneRenderer->Frame();
         m_Passes.push_back(CreateScope<Passes::ShadowPass>(
             *m_SceneGpu, frame, m_ShadowMaps));
+        // Skybox = HDRColor 的 first writer（清屏 + 画背景），先于 Forward 注册
+        m_Passes.push_back(CreateScope<Passes::SkyboxPass>(
+            *m_Device, *m_ShaderManager, *m_DescriptorManager, frame,
+            m_HDRColor, m_Environment.get()));
         m_Passes.push_back(CreateScope<Passes::ForwardPass>(
             *m_SceneGpu, frame, m_HDRColor, m_Velocity, m_Depth, m_ShadowMaps));
         m_Passes.push_back(CreateScope<Passes::TAAPass>(