README: the I-beam analysis demo, driven by an agent over MCP

The GIF plays inline and links the MP4; both are generated by tools/demo-video and attributed.
The asset check now also covers .mp4 files.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

xtask/src/assets.rs

@@ -16,6 +16,7 @@ const ASSET_EXT: &[&str] = &[
     "woff",
     "woff2",
     "gif",
+    "mp4",
     "bmp",
     "webp",
     "wav",