fix: 阴影不可见——灯序依赖遍历顺序 + 光比失衡，新增帧截图调试能力

- 灯序修复（根因）：entt Each 遍历序不保证创建序，lights[0] 曾落到
  点光（params.x=1）→ shader 阴影分支整个跳过。收集后 stable_sort
  方向光排前，确立"lights[0] = CSM 阴影光源"契约
- 光比失衡：IBL 环境与点光淹没主光阴影。KeyLight 5.0 + exposure
  默认 0.6 + ambientColor 0.07；阴影区环境光同步衰减（简化遮蔽）
- ShadowPass depth bias 0.006（acne 均值形态 vs 推过头丢阴影的折中）
- Renderer 帧截图能力：第 60 帧自动截 HDRColor/ShadowMap 落 BMP
  （ACES+gamma 还原屏幕观感），视觉调试自闭环不再依赖肉眼
- KeyLight 增加 intensity slider（0-20 运行时可调）

Engine/src/RenderGraph/RenderGraph.cpp

@@ -694,6 +694,24 @@ namespace Engine::RG {
         return entry.physical[frameSlot]->view;
     }
 
+    bool RenderGraph::GetCaptureTarget(TextureHandle h, u32 frameSlot,
+                                       vk::Image& outImage, vk::Extent2D& outExtent,
+                                       vk::Format& outFormat,
+                                       vk::ImageLayout& outResting) const
+    {
+        const auto& entry = GetEntry(h);
+        if (entry.imported || frameSlot >= entry.physical.size() ||
+            !entry.physical[frameSlot]) {
+            return false;
+        }
+        const auto& phys = *entry.physical[frameSlot];
+        outImage = phys.image;
+        outExtent = phys.extent;
+        outFormat = phys.format;
+        outResting = entry.restingLayout;
+        return true;
+    }
+
     void RenderGraph::SetQueryPool(vk::QueryPool pool, u32 queriesPerFrame)
     {
         m_QueryPool = pool;