D-10 (ruled 2026-10-02): a Q48.16 value is two cells at every cell
width, so every Q word is the same double word on 32- and 64-bit nodes.
Recorded in DECOMPOSITION.md section 3 and 5.26; the open note on the
Q.+/Q.- row is resolved by it.
Q.ABS and Q.NEG are the DABS and DNEGATE words. Checked against v3's
q48_abs and 0 - q on the 15 edge Q values and 20000 pseudo-random
values, at 32- and 64-bit cells, optimised and ASan+UBSan:
- 32-bit cells: bit-for-bit v3, including v3's wrap of Q min to itself.
- 64-bit cells: the low cell is v3's; the high cell is the true sign
(so ABS and NEG of Q min give +2^63 rather than wrapping), per D-10.
Breaking DNEGATE's carry fails Q.NEG, Q.ABS and Q.- checks.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
