The W17b/c sleep-stage migration widened the measurement unique
index to (user_id, type, measured_at, source, sleep_stage) with
NULLs treated as not-distinct, so three same-instant APPLE_HEALTH
weight rows now collide on insert. Derive a stable per-externalId
minute offset and stagger the seeded `measuredAt` accordingly so
the deletion-by-externalId contract is exercised without piggy-
backing on dedup behaviour the route doesn't care about.