fix: UBO descriptor visible to fragment shader — point light now works

Root cause: descriptor set layout binding had stageFlags=Vertex only.
Fragment shader needs pointLightPos/pointLightColor from UBO but couldn't
access it. Changed to Vertex|Fragment.

src/Engine.Graphics.Vulkan/VulkanPipeline.cs

@@ -152,7 +152,7 @@ internal sealed unsafe class VulkanPipeline : IDisposable
 
             var pushConstantRange = new VkPushConstantRange
             {
-                stageFlags = VkShaderStageFlags.Vertex,
+            stageFlags = VkShaderStageFlags.Vertex | VkShaderStageFlags.Fragment,
                 offset = 0,
                 size = 64,
             };