feat(v4.0.0): full-range UM* under plain F18 +* (D-3)

UM* as first written in DECOMPOSITION.md section 4 was exact only while
u1 <= 2^(n-2): plain +* loses the carry out of T and its shift keeps T's
sign bit, so the loop is exact only while S and T stay in
[-2^(n-2), 2^(n-2)).

The rewrite multiplies by s = u1 2/, which always lies in that range,
starting T at t0 = u2 2/ when u1 is odd, so the loop yields
hi:lo = t0 + s*u2 exactly. Then
  u1*u2 = 2*(hi:lo) + c_lo + c_hi*2^n
  c_lo  = u1 & u2 & 1
  c_hi  = (u1<0 ? u2 : 0) + (u1 odd and u2<0 ? 1 : 0)
restores the halved-away bits and the unsigned reading of both top bits.

test_foundation.c runs the new definition against v4_umul over every
pair of the edge vectors plus 20000 pseudo-random pairs, at 32- and
64-bit cells, optimised and under ASan+UBSan. The two pinned failing
cases are now ordinary exactness checks. Two hand mutations of the
correction step each fail more than 10000 checks at both widths.

DECOMPOSITION.md: section 4 UM* replaced, D-3 ruling text updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

v4/include/v4/umul.h

@@ -1,6 +1,7 @@
 /* umul.h -- unsigned multiply fixing UM* under D-3 constraints.
  *
- * DECOMPOSITION.md line 205-207: "NEEDS REVISION: written for the carry-keeping
+ * DECOMPOSITION.md section 4 first said of UM* (since rewritten to be exact over
+ * the full range; see test_foundation.c): "NEEDS REVISION: written for the carry-keeping
  * +* that D-3 rejected. With plain F18 +* the carry out of T is lost, so this
  * is only correct when the partial sums never overflow T (operands below 2^31).
  * A full-range UM* needs a correction step; to be rewritten and proven on the