fix: move light data from UBO to push constants — fixes std140 layout issue

Root cause: GLSL std140 layout adds padding after mat4 for vec4 fields,
causing light data to be read at wrong offsets → NaN/artifacts.

Fix:
- UBO back to 64 bytes (mat4 vp only)
- Light data (pos + color, 32 bytes) sent via push constants at offset 64
- Two push constant ranges: Vertex (0-64, model) + Fragment (64-96, light)
- Both vertex and fragment shaders read from same push_constant block
- PBR shader fully restored with Cook-Torrance BRDF
- Dynamic point light follows physics ball

src/Engine.Graphics.Vulkan/VulkanFrameResources.cs

@@ -5,7 +5,7 @@ namespace Engine.Graphics.Vulkan;
 internal sealed unsafe class VulkanFrameResources : IDisposable
 {
     public const int MaxFramesInFlight = 2;
-    public const ulong UboSize = 128;
+    public const ulong UboSize = 64;
 
     public VkCommandPool CommandPool;
     public VkCommandBuffer[] CommandBuffers = new VkCommandBuffer[MaxFramesInFlight];