progress_text(
text,r,g) ->handle
Start an animated progress indicator rendering text as a braille-masked cycling string
Returns a handle with a handle.stop() method that kills the animation
and cleans up. Each frame re-renders text via std.txt.mask_with_braille,
then erases it with backspaces before the next frame. The optional r
and g values anchor the hue so the animation stays in one color family;
omit them for per-frame-randomized colors.