fix: rendering now works — three critical bugs fixed

1. VkColorComponentFlags: wrong bit values (0x10-0x80 instead of 0x01-0x08)
   → colorWriteMask was 0xF0 instead of 0x0F → no color channels written

2. VkClearValue: was Sequential layout instead of Explicit (union)
   → depthStencil clear value written at wrong offset → depth buffer
   cleared to 0.0 instead of 1.0 → all fragments failed depth test

3. Vulkan clip space correction in vertex shader:
   gl_Position.y = -gl_Position.y (Vulkan Y-down vs OpenGL Y-up)
   gl_Position.z = (gl_Position.z + gl_Position.w) * 0.5 (Z [0,1] vs [-1,1])

Also:
- PushConstantSize fixed from 128 to 144 (two mat4 + vec4)
- Dynamic viewport/scissor state added to pipeline
- Push constants use column_major (default) to match System.Numerics row-major layout
- Camera.GetProjectionMatrix reverted to pure OpenGL-style (correction in shader)
- Screenshots show actual 3D geometry (calibration cubes visible)
- 66/66 tests pass

src/Engine.Graphics.Vulkan/VulkanRenderer.cs

@@ -475,7 +475,7 @@ public sealed unsafe class VulkanRenderer : IRenderer, IScreenshotProvider
         };
         Vk.vkCmdSetScissor(cmd, 0, 1, &scissor);
 
-        VkDescriptorSet ds = _descriptorSets[_currentFrame];
+        var ds = _descriptorSets[_currentFrame];
         Vk.vkCmdBindDescriptorSets(cmd, 0, _pipeline.PipelineLayout, 0, 1, &ds, 0, null);
 
         world.Each((Entity e, ref Transform transform, ref Mesh mesh, ref Material material) =>
@@ -502,7 +502,7 @@ public sealed unsafe class VulkanRenderer : IRenderer, IScreenshotProvider
                 proj = cam.GetProjectionMatrix();
             });
 
-            var mvp = model * view * proj;
+            var mvp = proj * view * model;
 
             var pushData = new byte[VulkanPipeline.PushConstantSize];
             fixed (byte* pPush = pushData)