StatickeepCurrent keep on a mesh's material (1 when unthinned / no uniform).
Tracker visit hook: re-derive this node's keep fraction from its fresh density record, apply a step change to material + brightness, and re-assert the uniform. Off-screen nodes hold their step (no churn on nodes nobody sees; they re-evaluate the frame they return).
Undo this node's thinning (guard turned off at runtime): keep back to 1, brightness recomputed without the density term, uniform cleared. A no-op on anything that is not a thinned data mesh.
True once since the last call if any node changed step (or drifted).
Owns the per-node keep ladder; driven by the tracker's
onVisithook.