Add a before/after screenshot of the stock editor to the README #22

The README's "The problem" section describes programming the Twister by hand in the MIDI Fighter Utility: click a knob, set its CC value, set its on and off color, one by one for every knob. A screenshot would show that instead of describing it.

Carried over from #21, where it was the one item left unchecked.

What to capture

• The stock MIDI Fighter Utility with one knob selected, showing what clicking through 16 knobs looks like.
• Optionally, a before/after pair: the editor on the left, the one-sentence chat request that replaces it on the right. The editor screenshot alone would also work.

Where it goes

At the end of "The problem" in README.md. The spec is the before-after.png entry under TODO in docs/images/README.md.

How to add it

• Host it on R2 under the mft-api/ prefix, like the hero GIF and the Twister map, not committed to the repo.
• Add the README reference in the same commit that makes the image available, so it never renders as a broken image.
• Check the capture for anything that shouldn't be public: file paths, window titles, other projects.
• Move the entry from TODO to "In the README" in docs/images/README.md.