Constant DEFAULT_FREEZE_FACTOR 

pub const DEFAULT_FREEZE_FACTOR: u32 = 2;

The default reduction factor η for the extrapolated-asymptote promotion rule: keep the top half of the field by predicted asymptote, freeze the rest.

Named distinctly from AshaPruner’s DEFAULT_REDUCTION_FACTOR because freeze-thaw surveys aggressively (a factor of two decides with a single competitor), where ASHA’s default keeps the top third.