fix: UBO binding 0 visible to fragment shader — lights now work

Root cause: descriptor set layout binding 0 (SceneUBO) had stageFlags=Vertex
only. Fragment shader reads numLights, lights[], shadowParams[] from UBO
but couldn't access it. Changed to Vertex|Fragment.

imgui.ini

@@ -28,3 +28,8 @@ Pos=772,456
 Size=507,263
 Collapsed=0
 
+[Window][Shadow & Light Parameters]
+Pos=-1,195
+Size=427,372
+Collapsed=0
+

src/Engine.Graphics.Vulkan/VulkanPipeline.cs

@@ -220,7 +220,7 @@ internal sealed unsafe class VulkanPipeline : IDisposable
             binding = 0,
             descriptorType = VkDescriptorType.UniformBuffer,
             descriptorCount = 1,
-            stageFlags = VkShaderStageFlags.Vertex,
+            stageFlags = VkShaderStageFlags.Vertex | VkShaderStageFlags.Fragment,
         };
         bindings[1] = new VkDescriptorSetLayoutBinding
         {