feat: shadow mapping — depth-only render pass from light POV + PCF

- VulkanShadowMap.cs: 2048x2048 D32_SFLOAT image, sampler (clamp-to-border),
  separate shadow pipeline (depth-only, no color, depth bias enabled, CULL_NONE)
- Shadow shaders: shadow.vert (lightViewProj * model * pos), shadow.frag (empty)
- Main shaders updated: fragLightSpacePos output from vertex, PCF 3x3 in fragment
- Push constants expanded to 160B: model(64) + lightPos(16) + lightColor(16) + lightViewProj(64)
- 3 push constant ranges: Vertex(model), Fragment(light), Vertex|Fragment(lightViewProj)
- Descriptor set: 2 bindings — UBO(vp) + CombinedImageSampler(shadowMap)
- Shadow pass: before main pass, renders scene depth from light position
- Image transitions: shadow map UNDEFINED→DEPTH→SHADER_READ_ONLY each frame
- vkCmdSetDepthBias(1.25, 0, 1.75) for acne prevention
- Light VP: CreateLookAt(lightPos, origin, up) * Perspective(60°, 1.0, 0.1, 60)
- 0 validation errors on build

src/Engine.Graphics.Vulkan/VulkanFrameResources.cs

@@ -140,19 +140,25 @@ internal sealed unsafe class VulkanFrameResources : IDisposable
 
     private void CreateDescriptorPoolAndSets(VkDescriptorSetLayout layout)
     {
-        var poolSize = new VkDescriptorPoolSize
+        var poolSizes = stackalloc VkDescriptorPoolSize[2];
+        poolSizes[0] = new VkDescriptorPoolSize
         {
             type = VkDescriptorType.UniformBuffer,
             descriptorCount = MaxFramesInFlight,
         };
+        poolSizes[1] = new VkDescriptorPoolSize
+        {
+            type = VkDescriptorType.CombinedImageSampler,
+            descriptorCount = MaxFramesInFlight,
+        };
 
         var poolInfo = new VkDescriptorPoolCreateInfo
         {
             sType = VkStructureType.DescriptorPoolCreateInfo,
             flags = VkDescriptorPoolCreateFlags.FreeDescriptorSet,
             maxSets = MaxFramesInFlight,
-            poolSizeCount = 1,
-            pPoolSizes = &poolSize,
+            poolSizeCount = 2,
+            pPoolSizes = poolSizes,
         };
 
         var descPool = VkDescriptorPool.Null;
@@ -204,6 +210,29 @@ internal sealed unsafe class VulkanFrameResources : IDisposable
         }
     }
 
+    public void UpdateShadowDescriptor(int frameIndex, VkSampler sampler, VkImageView shadowView)
+    {
+        var imageInfo = new VkDescriptorImageInfo
+        {
+            sampler = sampler,
+            imageView = shadowView,
+            imageLayout = VkImageLayout.ShaderReadOnlyOptimal,
+        };
+
+        var write = new VkWriteDescriptorSet
+        {
+            sType = VkStructureType.WriteDescriptorSet,
+            dstSet = DescriptorSets[frameIndex],
+            dstBinding = 1,
+            dstArrayElement = 0,
+            descriptorCount = 1,
+            descriptorType = VkDescriptorType.CombinedImageSampler,
+            pImageInfo = (nint)(&imageInfo),
+        };
+
+        Vk.vkUpdateDescriptorSets(_device, 1, &write, 0, 0);
+    }
+
     public void UpdateUbo(int frameIndex, void* data, ulong size)
     {
         void* pData = null;