fix: increase shadow bias — slope-dependent + larger depth bias

- Fragment shader: bias = 0.3 + 0.5 * (1 - NdotL) — slope-dependent
  Grazing angles get larger bias to prevent acne on cube faces
- vkCmdSetDepthBias: 2.5/0/3.5 (was 1.25/0/1.75) — more aggressive
  polygon offset for shadow map rendering
- Early exit tolerance reduced from 4x to 3x bias

src/Engine.Graphics.Vulkan/VulkanRenderer.cs

@@ -255,7 +255,7 @@ internal sealed unsafe class VulkanRenderer : IRenderer, Engine.Graphics.IScreen
             };
             Vk.vkCmdSetScissor(cmd, 0, 1, &shadowScissor);
 
-            Vk.vkCmdSetDepthBias(cmd, 1.25f, 0.0f, 1.75f);
+            Vk.vkCmdSetDepthBias(cmd, 2.5f, 0.0f, 3.5f);
 
             foreach (var dc in drawCalls)
             {