PresenceOverridesDao.create now refuses an override that overlaps any
existing override of the SAME child (inclusive boundaries): the check
runs inside the same transaction as the INSERT under the connection
lock, so a racing create cannot slip a conflict between check and
write — a deterministic race test proves exactly one of two concurrent
overlapping creates lands. The ValueError names the conflicting
override's id, its range, and its flag. Round-trip, per-child scope,
and remove-then-recreate tests added. Suite: 990 passed.
Joshua King
744a40dcb5