feat: 透明窗口 + 实例预算扩容（修复斜光阴影静默截断）

透明窗口：SceneSettings 加 ClearColor/DrawSkybox，随 FrameSceneData
每帧下发到 SkyboxPass（替换硬编码清屏色）；GLFW 透明帧缓冲已开，
tonemap 预乘 alpha 直通已有，链路全通。TAA alpha 时域混合为已知边界。

实例预算：kMaxInstances 16384 → 65536（四视图共享预算，100 实体
斜光时需求 16.4k 超限，cascade 2 实例被静默丢弃 = 用户看到的
"拖灯光方向部分背包阴影消失"）。截断状态每帧落遥测 + ImGui 红字
（原 warn-once 对动态穿越阈值的场景等于没有告警）。

Engine/src/Renderer/Passes/SkyboxPass.cpp

@@ -94,9 +94,10 @@ namespace Engine::Passes
             ZoneScopedN("Skybox");
             const vk::CommandBuffer cmd = ctx.cmd;
 
+            const glm::vec4& cc = m_Frame.ClearColor;
             vk::ClearValue clear;
             clear.color = vk::ClearColorValue(
-                std::array<float, 4>{0.1f, 0.1f, 0.1f, 1.0f});
+                std::array<float, 4>{cc.r, cc.g, cc.b, cc.a});
 
             vk::RenderingAttachmentInfo color;
             color.setImageView(ctx.View(m_HDRColor))
@@ -112,13 +113,13 @@ namespace Engine::Passes
 
             cmd.beginRendering(renderingInfo);
 
-            if (m_Env) {
+            if (m_Env && m_Frame.DrawSkybox) {
                 // 无环境贴图：只清屏（Forward loadOp=Load 依赖这里先写）
-                std::array<vk::ShaderStageFlagBits, 2> stages = {
+                const std::array<vk::ShaderStageFlagBits, 2> stages = {
                     vk::ShaderStageFlagBits::eVertex,
                     vk::ShaderStageFlagBits::eFragment
                 };
-                std::array<vk::ShaderEXT, 2> shaders = {m_Vert, m_Frag};
+                const std::array<vk::ShaderEXT, 2> shaders = {m_Vert, m_Frag};
                 cmd.bindShadersEXT(stages, shaders);
 
                 cmd.bindDescriptorSets(vk::PipelineBindPoint::eGraphics,
@@ -149,12 +150,12 @@ namespace Engine::Passes
                 cmd.setVertexInputEXT({}, {});
 
                 cmd.setRasterizationSamplesEXT(vk::SampleCountFlagBits::e1);
-                vk::SampleMask sampleMask = 0xFFFFFFFF;
+                constexpr vk::SampleMask sampleMask = 0xFFFFFFFF;
                 cmd.setSampleMaskEXT(vk::SampleCountFlagBits::e1, sampleMask);
                 cmd.setAlphaToCoverageEnableEXT(VK_FALSE);
                 cmd.setAlphaToOneEnableEXT(VK_FALSE);
 
-                vk::Viewport viewport(0.0f, 0.0f,
+                const vk::Viewport viewport(0.0f, 0.0f,
                                       static_cast<float>(ctx.extent.width),
                                       static_cast<float>(ctx.extent.height),
                                       0.0f, 1.0f);
@@ -170,8 +171,8 @@ namespace Engine::Passes
                              .setDstAlphaBlendFactor(vk::BlendFactor::eZero)
                              .setAlphaBlendOp(vk::BlendOp::eAdd);
                 // ArrayProxyNoTemporaries 拒绝临时量：具名局部变量传入
-                vk::Bool32 blendEnable = VK_FALSE;
-                vk::ColorComponentFlags colorWriteMask =
+                constexpr vk::Bool32 blendEnable = VK_FALSE;
+                constexpr vk::ColorComponentFlags colorWriteMask =
                     vk::ColorComponentFlagBits::eR |
                     vk::ColorComponentFlagBits::eG |
                     vk::ColorComponentFlagBits::eB |