Quantized horizontal coordinate, [0, QM_MAX] across the parent's [west, east].
Quantized vertical coordinate, [0, QM_MAX] across the parent's [south, north].
Quantized height, [0, QM_MAX] across [minHeight, maxHeight].
Surface (skirt-free) triangle indices into u/v/height.
The minimal parent-surface data upsampleQuantizedMesh reads: quantized u/v/height plus the surface triangle indices and the parent's height range (to de-quantize heights into meters before re-quantizing to the child's own range). These are exactly the arrays a TerrainHeightSampler already retains.