Review round 4: turning the mask's grey into opacity dropped the partial
coverage at cone edges, and a painter composing its cone in an inner layer
restored it with srcOver, past the lighten. The mask now starts opaque
black, so a half-covered edge pixel is half as grey, and each cone is
painted in its own layer that is lightened in whole.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>