Walk the spinner slowly around a rainbow gradient

The default AnimatedMarker stepped a full stop of a three-colour
gradient on every redraw (cyan, magenta, yellow, then a hard reset),
so its colour jumped. It now advances twelve degrees of hue per redraw
around a new HueGradient, exposed as colors.rainbow, and completes a
lap every 30 redraws. The lap length is configurable through the new
color_cycle argument, which rejects values below 1.

HueGradient wraps at the seam (0 and 1 are both red) and emits colours
without an xterm index, so truecolor terminals render the exact RGB and
256-colour terminals the nearest cube entry instead of snapping to a
registered endpoint.

Re-render the demos that show the default spinner and regenerate the
API surface snapshot for the widened AnimatedMarker signature.

Author: wolph