fix: increase shadow cubemap resolution to 2048 — sharper edges

- ShadowMapSize: 1024 → 2048 (per face, 6 faces)
- 4x more texels per face — less aliasing on cube edges
- Memory: 6 * 2048² * 4B (R32) + 6 * 2048² * 4B (D32) = ~192MB

src/Engine.Graphics.Vulkan/VulkanShadowMap.cs

@@ -5,7 +5,7 @@ namespace Engine.Graphics.Vulkan;
 
 internal sealed unsafe class VulkanShadowMap : IDisposable
 {
-    public const uint ShadowMapSize = 1024;
+    public const uint ShadowMapSize = 2048;
     public const float FarPlane = 60.0f;
 
     public VkImage ColorImage;