The selector returned only the arrows it would draw, so a view could not draw
anything at a quiet sample and the below-threshold branch was unreachable. It
now returns every candidate location plus an `active` flag, both callers filter
on it, and the magnitude colour ramp is fixed so one magnitude is one colour
across frames. Verified by pixel parity against HEAD for the 2D canvas and the
3D overlay, and by the shared and studio suites.
ykcheungab
ca28d788c2