Review round 3: cones of different strengths (one fading in, one out)
still stacked, with a jump where their strengths met. Each cone now paints
its strength as an opaque grey with a lighten blend, so an overlap takes the
strongest, and the mask turns grey into opacity. While the replay's cone
worker starts or after it fails, the agents' own cones now join a coverage
layer too instead of painting stacked grey.

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