Place records every child's slot; GenerateEntities marks belt candidates. After the swarm pass, a lane pass on its own
stream gives a candidate that kept its body, has a band at least the lane's half-width on both sides, carries no swarm and
whose parent has no other belt shape, an annulus on a copy of its rail: rocks over [slot inner, D - E] and [D + E, slot
outer], E = LaneBodyRadii body radii. ZoneGenerationSettings gains LaneProbability (0 in code, 0.5 authored) and
LaneBodyRadii (5). LanedBeltTests pins it; EveryBeltIsKindedAtGeneration moves there and asserts lanes; BeltShapes.Lanes
lands with it; both TrojanSwarmTests fixtures draw no lane.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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