Constant MAX_FREQ_SCORE 

pub const MAX_FREQ_SCORE: u64 = FREQ_MASK; // 1_048_575u64

Largest freq_score the packed value can represent (2^FREQ_BITS - 1). The single source of truth for the freq domain — tests and any caller that needs to clamp/validate import this rather than hardcoding a copy (a stale copy in the proptest survived the E1 FREQ_BITS 56→20 change and silently broke the invariants until proptest caught it).