Zone.Vent is the one adder, StepClouds the one remover, Zone.Obscuration the
one reader: the product of (1 - opacity) over clouds whose disc the segment
touches (closest point of the segment to the centre within Radius), 1f when
none. Derived from live clouds at each call, never cached. VapourDump vents
once on its first run, drifting with its venter. Nothing reads Obscuration
yet (Sensor.Gain takes it in the next commit); this commit builds.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
6dcf33d11c