Function select_due_scheduled_times 

pub fn select_due_scheduled_times(
    schedule: &EvalSchedule,
    cursor_secs: i64,
    wall_now_secs: i64,
) -> Result<DueScheduledTimes>

Select due scheduled times <= wall_now_secs without materializing all missed ticks.

Fallible: a non-positive interval or a scheduled time that does not fit in i64 yields an explicit error instead of silently producing saturated non-phase timestamps.