computeSleepStageBreakdown keyed each stage row by the calendar day of
its own measuredAt and never collapsed multi-source, so a night where
sleep began before midnight split across two perNight buckets and a
WHOOP + Apple Health night double-counted every stage. The stacked-bar
therefore disagreed with the dashboard tile and the chart series for the
same dates.
Route it through the canonical reconstructSleepNights: session-cluster
the stages, key by the wake day, collapse to one canonical source, and
apply the granular-over-bare de-dup. The stacked-bar now agrees with the
rest of the sleep surface.