Reformulate composeRegression from the determinant form (n*Sxx - Sx^2)
to the mean-centered corrected-sum identities (Sxx = Sxx - Sx^2/n, ...).
Algebraically identical to the prior form and to Postgres REGR_* /
STDDEV_POP, but it subtracts the x mean at matched scale and avoids the
catastrophic cancellation the determinant form suffers on the ~1e10
epoch-day x-axis. Also read the exact stored sum_value accumulator for
the Sy term instead of reconstructing it as mean*count, dropping the
AVG-to-multiply float residual; fall back to mean*count when the column
is null.
Composition-only: no migration, no schema change. The WEIGHT live-parity
case stays bit-identical at the 1e-9 relative bound. The near-flat DST
parity seed improves by ~30x but still exceeds 1e-9, so it remains
quarantined with an updated note pointing at a window-local x-rescale as
the real fix.
Marc-André Bombeck
672e3f0e07