Constant BULK_FILL_MIN_LEN 

const BULK_FILL_MIN_LEN: usize = 64;

Minimum sub-range length before the bulk-fill fast path in write_leaf becomes profitable for null-heavy leaf columns. Below this, per-call slice + popcount overhead regresses list/struct paths that call write_leaf many times with tiny ranges. Picked via threshold sweep; see https://github.com/apache/arrow-rs/pull/9967 for the rationale.