RecomputeTargetSet sampled TerrainSampler.HeightAt (a 169-point biome
blend kernel) 5x per column over the whole render distance on the main
thread every chunk crossing (~1,445 calls = ~250k biome lookups), causing
multi-frame hitches while moving. Memoize each chunk column's (min,max)
surface range, reused across ticks and pruned to the horizontal window, so
each column is sampled once. Exact (no terrain change); spike is amortized
to a thin edge strip per crossing and zero when stationary.
9c28bf152f