DECOMPOSITION.md 5.26 makes Q.+ and Q.- the D+ and D- words. They are
checked against v3's q48_add/q48_sub (uint64_t a + b, a - b, wrapping)
on every pair of 15 edge Q values (0, ulp, 0.5, 1.0, 1.5, -1.0, -ulp,
values across the 32-bit seam, Q max and min, +-12345.0) and 20000
pseudo-random pairs, 2682 of which overflow Q48.16.
- 32-bit cells: bit-for-bit v3's result, overflow wrap included.
- 64-bit cells, with a Q value as a sign-extended double: the low cell is
v3's result; on overflow the high cell holds the true carry where v3
wraps. Whether a Q value is one cell or two on a 64-bit node is not
ruled; recorded as open in 5.26.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
