docs: add demo GIF to README

- cortex_demo.gif: 400px, 10fps, palette-optimized (833KB)
- Converted from cortex.mp4 (3.8s, 1280x720, 30fps)
- Shows physics simulation with PBR + shadows + multi-light
- README.md: embedded GIF at top
- .gitignore: added cortex.mp4 (source video, too large for git)

.gitignore

@@ -37,3 +37,4 @@ imgui.ini
 .playwright-mcp/
 Vulkan-Guide/
 cortex.mp4
+cortex.mp4

README.md

@@ -2,6 +2,8 @@
 
 AI-Native 3D game engine with pure P/Invoke Vulkan 1.3 render backend.
 
+![Cortex Engine Demo](cortex_demo.gif)
+
 ## Features
 
 - **Vulkan 1.3** — dynamic rendering, synchronization2, pure P/Invoke (no wrapper libraries)