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/VulkanPipeline.cs

@@ -150,7 +150,7 @@ internal sealed unsafe class VulkanPipeline : IDisposable
                 pDynamicStates = dynamicStates,
             };
 
-            var pushConstantRanges = stackalloc VkPushConstantRange[2];
+            var pushConstantRanges = stackalloc VkPushConstantRange[3];
             pushConstantRanges[0] = new VkPushConstantRange
             {
                 stageFlags = VkShaderStageFlags.Vertex,
@@ -163,6 +163,12 @@ internal sealed unsafe class VulkanPipeline : IDisposable
                 offset = 64,
                 size = 32,
             };
+            pushConstantRanges[2] = new VkPushConstantRange
+            {
+                stageFlags = VkShaderStageFlags.Vertex | VkShaderStageFlags.Fragment,
+                offset = 96,
+                size = 64,
+            };
 
             var descLayout = DescriptorSetLayout;
             var layoutInfo = new VkPipelineLayoutCreateInfo
@@ -170,7 +176,7 @@ internal sealed unsafe class VulkanPipeline : IDisposable
                 sType = VkStructureType.PipelineLayoutCreateInfo,
                 setLayoutCount = 1,
                 pSetLayouts = &descLayout,
-                pushConstantRangeCount = 2,
+                pushConstantRangeCount = 3,
                 pPushConstantRanges = (nint)pushConstantRanges,
             };
 
@@ -221,19 +227,27 @@ internal sealed unsafe class VulkanPipeline : IDisposable
 
     private void CreateDescriptorSetLayout()
     {
-        var binding = new VkDescriptorSetLayoutBinding
+        var bindings = stackalloc VkDescriptorSetLayoutBinding[2];
+        bindings[0] = new VkDescriptorSetLayoutBinding
         {
             binding = 0,
             descriptorType = VkDescriptorType.UniformBuffer,
             descriptorCount = 1,
             stageFlags = VkShaderStageFlags.Vertex,
         };
+        bindings[1] = new VkDescriptorSetLayoutBinding
+        {
+            binding = 1,
+            descriptorType = VkDescriptorType.CombinedImageSampler,
+            descriptorCount = 1,
+            stageFlags = VkShaderStageFlags.Fragment,
+        };
 
         var info = new VkDescriptorSetLayoutCreateInfo
         {
             sType = VkStructureType.DescriptorSetLayoutCreateInfo,
-            bindingCount = 1,
-            pBindings = &binding,
+            bindingCount = 2,
+            pBindings = bindings,
         };
 
         var descLayout = VkDescriptorSetLayout.Null;