Record the demo GIF and embed it in the README

[Silousr]

make demo is the most convincing thing in the repo and the README has a placeholder where its recording should go.

• brew install vhs && vhs docs/demo.tape produces docs/demo.gif (the tape is timing-accurate: the demo completes in about 5 s).
• Replace the placeholder comment near the top of README.md with ![labwire closed-loop demo](docs/demo.gif).
• Keep the file under ~3 MB so the README stays fast.

See docs/demo-gif.md.