The per-bucket OLS accumulators stored x on the raw epoch-day axis (x ~ 20540),
so sum_xx accumulated past ~1e10 and the square shed ~10 of a double's
significant digits before the value was ever stored. The sub-day x detail was
lost at accumulation time, which no read-side identity can recover; an
ill-conditioned window's composed slope/r2 then drifted from the live REGR_*
probe past the parity gauge.
Rebase x to a fixed origin (epoch-days of 2020-01-01) at write time so the
squared terms stay O(1e7) and exact. Slope, r2, and population sd are invariant
under the affine x-shift, so the live raw-epoch probe still parity-matches the
cross-bucket compose; composeRegression needs no formula change. The per-bucket
slope/r2 columns stay on the raw axis (shift-invariant).
Migration 0193 recomputes sum_x/sum_xy/sum_xx from measurements on the rebased
basis, one set-based update per granularity. Recompute-from-source is idempotent
and rerun-safe; it touches only rows that already carry the accumulators and
leaves pre-migration NULLs for the boot re-fold.
Un-quarantine the DST-boundary parity test: it now matches live REGR_* within
the existing 1e-9 relative bound, with the WEIGHT case still bit-identical.
Marc-André Bombeck
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