The brightness column changes and commits in one handler, so the commit
read the render's draft (still null) and sent nothing; the queued draft then
stuck and rode along with the next release. The pending colour lives in a ref
now. color-drag.spec gains a column drag.
The wheel's arithmetic is checked in color.check.ts; nothing checked what a
drag across it sends. A new Playwright spec drags from the centre out to
three o'clock, counts the panel's publishes off the wire, and holds the disc
to one message at the release rather than one per pixel.