fix: video recording — proper layout transitions + deferred buffer read

Three validation errors fixed:
1. vkCmdCopyImageToBuffer inside render pass → moved AFTER vkCmdEndRendering
2. Swapchain images missing TRANSFER_SRC usage → added to swapchain creation
3. Wrong layout transitions → ColorAttachment→TransferSrc→copy→ColorAttachment

Architecture change:
- CaptureFrame now only records commands (returns null)
- ReadCapturedBuffer called at start of NEXT frame (after WaitFrame/fence)
  GPU has finished by then, safe to map memory
- CapturedFrame available 1 frame late (acceptable for video)
- Image transitions: ColorAttachmentOptimal→TransferSrcOptimal→ColorAttachmentOptimal
  (present transition still works because image is back in ColorAttachmentOptimal)

src/Engine.Graphics.Vulkan/VulkanRenderer.cs

@@ -107,10 +107,10 @@ public sealed unsafe class VulkanRenderer : IRenderer, Engine.Graphics.IScreensh
         var w = _swapchain.Extent.Width;
         var h = _swapchain.Extent.Height;
 
-        // Transition swapchain image: PresentSrc → TransferSrc
+        // Transition: ColorAttachmentOptimal → TransferSrcOptimal
         TransitionImageLayout(cmd, _swapchain.Images[imageIndex],
-            VkImageLayout.PresentSrcKHR, VkImageLayout.TransferSrcOptimal,
+            VkImageLayout.ColorAttachmentOptimal, VkImageLayout.TransferSrcOptimal,
-            0x8000, 0, 0x10000, 0x2000);
+            0x400, 0x100, 0x10000, 0x2000);
 
         // Copy image to buffer
         var region = new VkBufferImageCopyRegion
@@ -132,12 +132,18 @@ public sealed unsafe class VulkanRenderer : IRenderer, Engine.Graphics.IScreensh
         Vk.vkCmdCopyImageToBuffer(cmd, _swapchain.Images[imageIndex], VkImageLayout.TransferSrcOptimal,
             _screenshotBuffer, 1, &region);
 
-        // Transition back: TransferSrc → PresentSrc
+        // Transition back: TransferSrcOptimal → ColorAttachmentOptimal
         TransitionImageLayout(cmd, _swapchain.Images[imageIndex],
-            VkImageLayout.TransferSrcOptimal, VkImageLayout.PresentSrcKHR,
+            VkImageLayout.TransferSrcOptimal, VkImageLayout.ColorAttachmentOptimal,
-            0x10000, 0x2000, 0x8000, 0);
+            0x10000, 0x2000, 0x400, 0x100);
+
+        return null; // Data will be read next frame after GPU completes
+    }
+
+    private byte[]? ReadCapturedBuffer()
+    {
+        if (!_screenshotInitialized) return null;
 
-        // Map and read
         void* pData = null;
         Vk.vkMapMemory(_ctx.Device, _screenshotMemory, 0, _screenshotBufferSize, 0, &pData);
 
@@ -265,6 +271,12 @@ public sealed unsafe class VulkanRenderer : IRenderer, Engine.Graphics.IScreensh
     {
         _frameResources.WaitFrame(_frameIndex);
 
+        // Read captured frame from previous render (GPU has finished by now)
+        if (IsRecording)
+        {
+            CapturedFrame = ReadCapturedBuffer();
+        }
+
         uint imageIndex;
         var acquireResult = Vk.vkAcquireNextImageKHR(_ctx.Device, _swapchain.Swapchain,
             ulong.MaxValue, _frameResources.AcquireSemaphores[_frameIndex], VkFence.Null, &imageIndex);
@@ -505,16 +517,16 @@ public sealed unsafe class VulkanRenderer : IRenderer, Engine.Graphics.IScreensh
             Vk.vkCmdDrawIndexed(cmd, dc.indexCount, 1, 0, 0, 0);
         }
 
-        // Capture frame BEFORE ImGui (video without UI)
+        _imGui?.Render(cmd, _swapchain.Extent.Width, _swapchain.Extent.Height);
+
+        Vk.vkCmdEndRendering(cmd);
+
+        // Capture frame AFTER render pass ends, BEFORE present transition
         if (IsRecording)
         {
             CapturedFrame = CaptureFrame(cmd, imageIndex);
         }
 
-        _imGui?.Render(cmd, _swapchain.Extent.Width, _swapchain.Extent.Height);
-
-        Vk.vkCmdEndRendering(cmd);
-
         TransitionImageLayout(cmd, _swapchain.Images[imageIndex],
             VkImageLayout.ColorAttachmentOptimal, VkImageLayout.PresentSrcKHR,
             0x400, 0x100,

src/Engine.Graphics.Vulkan/VulkanSwapchain.cs

@@ -78,7 +78,7 @@ internal sealed unsafe class VulkanSwapchain : IDisposable
             imageColorSpace = _surfaceFormat.colorSpace,
             imageExtent = Extent,
             imageArrayLayers = 1,
-            imageUsage = VkImageUsageFlags.ColorAttachment | VkImageUsageFlags.TransferDst,
+            imageUsage = VkImageUsageFlags.ColorAttachment | VkImageUsageFlags.TransferDst | VkImageUsageFlags.TransferSrc,
             imageSharingMode = VkSharingMode.Exclusive,
             preTransform = caps.currentTransform,
             compositeAlpha = VkCompositeAlphaFlagsKHR.Opaque,