Function worked_secs 

fn worked_secs(mine: &[&TaskTransition], now: SystemTime) -> u64

The sum of a row’s in_progress intervals, from its transitions.

mine is that one row’s transitions, in time order. They are grouped and sorted once for the whole board rather than filtered per row: the log is read on every board read and every task write, and scanning it once per row makes each of those O(rows x history).