Function any_unclaimed 

fn any_unclaimed(
    reverse_bits: &[u64],
    forward_bits: &[u64],
    counter: u64,
) -> bool

Whether any reverse key’s short id went unclaimed by a forward value.

The bitmaps round up to a whole word, so ids past the counter are addressable in the last one and are masked off. The mask keeps the counter’s own bit, matching the bound the loser pass applies.