fix: imageCubeArray via core features + SRGB swapchain + LUT color conversion

Three fixes:

1. imageCubeArray: enabled via VkPhysicalDeviceFeatures (core 1.0 feature)
   instead of VkPhysicalDeviceImageCubeArrayFeatures (wrong sType in pNext
   chain was interpreted as VkExternalMemoryImageCreateInfoNV)

2. Swapchain format: reverted to B8G8R8A8_SRGB (was UNORM which driver
   didn't support → fell back to A2B10G10R10 10-bit format=58 → wrong
   pixel layout for capture)

3. Video colors: SRGB→linear conversion via 256-entry LUT in
   ReadCapturedBuffer. SRGB swapchain gives gamma-compressed bytes,
   FFmpeg expects linear. LUT converts each B/G/R channel using
   standard SRGB formula: s<=0.04045 ? s/12.92 : ((s+0.055)/1.055)^2.4

Also: video captures before ImGui (no UI in recording)

src/Engine.Graphics.Vulkan/VulkanRenderer.cs

@@ -149,10 +149,35 @@ public sealed unsafe class VulkanRenderer : IRenderer, Engine.Graphics.IScreensh
         Marshal.Copy((nint)pData, result, 0, (int)_screenshotBufferSize);
         Vk.vkUnmapMemory(_ctx.Device, _screenshotMemory);
 
-        // Data is BGRA from swapchain — no conversion needed, FFmpeg handles bgra
+        // Swapchain is B8G8R8A8_SRGB — raw bytes are SRGB-encoded
+        // Convert SRGB→linear for FFmpeg (which expects linear bgra)
+        for (int i = 0; i < result.Length; i += 4)
+        {
+            result[i] = SrgbToLinear(result[i]);     // B
+            result[i + 1] = SrgbToLinear(result[i + 1]); // G
+            result[i + 2] = SrgbToLinear(result[i + 2]); // R
+            // result[i + 3] = A (keep as-is)
+        }
+
         return result;
     }
 
+    private static readonly byte[] SrgbLut = BuildSrgbLut();
+
+    private static byte[] BuildSrgbLut()
+    {
+        var lut = new byte[256];
+        for (int i = 0; i < 256; i++)
+        {
+            float s = i / 255.0f;
+            float l = s <= 0.04045f ? s / 12.92f : MathF.Pow((s + 0.055f) / 1.055f, 2.4f);
+            lut[i] = (byte)Math.Clamp(l * 255.0f, 0, 255);
+        }
+        return lut;
+    }
+
+    private static byte SrgbToLinear(byte srgb) => SrgbLut[srgb];
+
     public void BeginImGuiFrame()
     {
         _imGui?.NewFrame();