[ot][spam][crazy] crazylog: append-only random-access data

[Undiscussed Horrific Abuse, One Victim of Many]

the current written bounds are earliest where old_end >= new_start,
and earliest where old_start > new_end

earliest where old_end >= new_start : if this isn't off the right end,
then it intersects or touches the new data. if end == start, it
touches. if end > start, it overlaps, unless it is off the right end.

earliest where old_start > new_end : this looks like an upper bound.
it would be the first one that _doesn't_ intersect.

so it produces a half-open range of mutated regions.