Function routines_matching 

pub fn routines_matching<'a>(
    routines: &'a [Routine],
    kind: BreakKind,
    categories: &[RoutineCategory],
    max_difficulty: RoutineDifficulty,
) -> Vec<&'a Routine>

The routines that match a break kind and the engine filters: the kind’s pool, intersected with categories (empty means “all categories”) and capped at max_difficulty. Pure so every filter combination is unit-testable. Sleep matches nothing.