Per-density-level hash cutoffs, indexed [level - 1][tier]: a slot whose
spawn hash falls below thresholds[level-1][tier] (and above the previous
tier's) draws that tier. The default table fills 40 / 60 / 80 % of slots
for levels 1 / 2 / 3, with the dense tier only reached at level 3.
Per-density-level hash cutoffs, indexed
[level - 1][tier]: a slot whose spawn hash falls belowthresholds[level-1][tier](and above the previous tier's) draws that tier. The default table fills 40 / 60 / 80 % of slots for levels 1 / 2 / 3, with the dense tier only reached at level 3.