feat: PBR point light — Unity-style attenuation, warm glow

- UBO expanded from 64 to 128 bytes: vp(64) + lightPos(16) + lightColor(16)
- Vertex shader: passes point light UBO data through
- Fragment shader: refactored calcLight() function, shared by directional + point
  - Point light: Unity-style attenuation pow(1 - dist/range, 2)
  - Directional light reduced intensity (0.4) to balance with point light
  - Point light: position (0,8,0), warm color (1,0.9,0.7), intensity 15, range 25
- Renderer: reads Light component from ECS, packs into UBO with Marshal.StructureToPtr
- Scene: MainLight entity with Light.Point at (0,8,0)
- 0 C# struct changes — pure UBO layout + shader upgrade

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 = 64;
+    public const ulong UboSize = 128;
 
     public VkCommandPool CommandPool;
     public VkCommandBuffer[] CommandBuffers = new VkCommandBuffer[MaxFramesInFlight];