A cumulative metric has no meaningful latest reading — a partial day's step
count is a fragment, not a value — so the only honest comparison is against
the same person's own typical total at the same local hour. That needs an
hourly substrate and there was none: the nightly cumulative drain collapses
per-sample Apple Health rows into one daily total after the grace window and
hard-deletes the originals in the same transaction. Today's curve was
derivable; every older day's was already gone.
Fold the rows into twenty-four running totals and write them down first, in
the same transaction and before the delete. The merge is additive so a day
drained twice keeps both halves, and the rows are re-read under the advisory
lock the drain already takes so an overlapping tick folds nothing.
One table, one nightly range delete to bound it.
