Constant PARALLEL_SAMPLES_THRESHOLD 

const PARALLEL_SAMPLES_THRESHOLD: usize = 64;

Minimum sample count to bother parallelising the per_point loop. Below this, rayon’s map_init overhead (per-worker MilkEvaluator clone + work-stealing dispatch) exceeds the eval time we’d save — stay sequential. 64 lines up roughly with the smallest “dense” waves we care about (the typical preset uses 256-512).