stillwater-sc / universal / 26345412711

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fix(ereal): IEEE 754 signed-zero subtraction (#962) (#964)

-0 - +0 returned +0 instead of -0. Two defects on the signed-zero path:

1. apply_ieee754_add_special_values() had no both-operands-zero branch, so
   the result fell through to the general renormalize path which collapses
   any zero to +0. Add the IEEE 754 round-to-nearest rule: a sum of two
   zeros is -0 only if both addends are -0. operator-= routes here on the
   sign-flipped RHS, so this single rule produces the correct subtraction
   table (-0 - +0 = -0 + -0 = -0; -0 - -0 = -0 + +0 = +0).

2. convert_to_ieee754() seeded its accumulator with +0, and IEEE +0 + -0
   == +0, discarding a stored -0 on conversion to native double. Seed from
   the leading (dominant) component instead -- numerically identical for
   non-zero expansions, sign-preserving for zero.

Unskip the -0 - +0 case in elastic/ereal/arithmetic/subtraction.cpp (drop
the EREAL_TEST_ISSUE_962 gate) and add the -0 - -0 = +0 case. Verified
foundational tests pass on both gcc-13 and clang-18.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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