Function generate_with_constraint 

pub fn generate_with_constraint<'g>(
    rows: RowFlow<'g>,
    constraint: Option<Constraint>,
) -> RowFlow<'g>

Trim trailing rows once the constraint stops matching — a break, NOT a filter (generateWithConstraint, memory-source.ts:505). Valid because the chosen index is sorted by the constraint columns first, so once the scan passes the last matching row every subsequent row fails (§3.1).