Grid region size in chunks — one candidate structure per region.
Jitter margin within a region in chunks (origin stays spacing - separation from the far edge).
Decorrelates this structure's region grid from others.
Decides and lays out a structure at a candidate origin chunk. Return null for "no structure here". Must be deterministic for a given (origin, ctx): it is recomputed by every chunk the structure overlaps.
A registered structure type.