UM/MOD is assembled exactly as DECOMPOSITION.md section 4 gives it and
needs no change: it is exact for every uhi < ud at 32- and 64-bit cells.
It is checked against q*d + r = uhi:ulo with r < d through v4_umul, so
no second C divider has to be trusted. Coverage: all edge-vector triples
with uhi < ud plus 20000 pseudo-random cases (top-bit, small and
near-maximum divisors), optimised and under ASan+UBSan. Two hand
mutations each fail more than 15000 checks.
New headroom probe: runs a word with marked cells under the canary and
under its return address and reports how many survive, since the D-2
circular stacks overwrite silently instead of faulting. Measured:
UM* 6 data cells under its args, 4 return entries under its return
UM/MOD 3 data cells under its args, 3 return entries under its return
DECOMPOSITION.md: UM/MOD marked executed, with its defined range and
stack limits.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
