stillwater-sc / universal / 34792158804

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fix(dfloat): follow IEEE 754 for the sign of zero (#1492)

dfloat broke IEEE 754's signed-zero rules in five places:
- A zero product or quotient was +0 whatever the operand signs: -0 * 2, 0 * -2, -0 / 2
  and 0 / -2 all gave +0. It now takes the XOR of the signs.
- operator+= returned its left operand when the right one was zero, so -0 + +0 stayed
  -0. The sum of two zeros is now -0 only when both are -0 (roundTiesToEven), and
  x + 0 is still x.
- An exact zero sum of opposite signs took the left operand's sign, so -2 + 2 was -0.
  It is now +0.
- operator-() skipped zeros, so -(+0) was +0. IEEE negate flips the sign of every
  value.
- operator-= negated its operand the same way, so -0 - -0 = -0 + -0 was -0. It now
  negates zeros too, which gives +0.
Division by infinity, infinity arithmetic, comparisons (-0 == +0) and the sign of an
underflowed result were already right and are unchanged.

New test static/float/dfloat/arithmetic/signed_zero.cpp (dfloat_signed_zero): 28 cases
(the table above, plus sums, differences, division by +-inf, underflow at minpos and
negation) for decimal32/64 in BID and DPD, decimal128, dfloat<4> and dfloat<5> DPD, with
the expected signs from Python's decimal module in the IEEE context. It fails every
configuration on the parent commit and passes on gcc 13.3 and clang with 0 warnings. All
62 dfloat, blockdecimal and dfixpnt test runs pass.

The full-range cross-check against Python's decimal contexts (366,079 parses and
+ - * / results over decimal32/64/128, BID and DPD, dfloat<4> and <5>) had found 1,782
zero results with the wrong sign and no wrong values. It now finds neither. In the dfloat
differential against the parent, only 64 lines change, each a zero whose sign flips.

Closes #1491

BEHAVIOUR-CHANGE: dfloat zero results now carry IEEE 754 signs: -0 * 2 and 0 / -2 are -0 (were +0), -0 + +0, -0 - -0 and -2 + 2 are +0 (were -0), and unary minus flips the sign of a zero

Co-Authored-By: Claude Opus 5 (1M context) ... (continued)

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