stillwater-sc / universal / 24956707653

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test(math): harden pow signed-zero and special-value test checks (CodeRabbit) (#773)

Two findings on PR #770 (already merged) that warrant a follow-up
test-hardening PR:

1. Signed-zero static_asserts cannot detect sign regressions.
   `+0.0 == -0.0` is true in IEEE-754, so `static_assert(value == -0.0)`
   passes whether value is +0 or -0. The implementation in pow.hpp
   correctly handles signed zero (it uses bit_cast to detect sign), but
   a future regression to the wrong-sign result would not have been
   caught by these tests.

   Fix: add constexpr-friendly sign_bit / is_pos_zero / is_neg_zero
   helpers in the test (using std::bit_cast since std::signbit isn't
   constexpr until C++23 and Universal targets C++20). Use these in
   the static_asserts that pin signed-zero contracts: pow(-inf, -3)
   == -0, pow(-inf, -4) == +0, pow(-0, 3) == -0, pow(-0, 4) == +0,
   pow(-0, 2.5) == +0, pow(+0, 3) == +0.

   ==-inf vs +inf comparisons remain via == because +inf != -inf in
   IEEE-754; only ==0.0 has the signed-zero-equality issue.

2. Runtime check lambda silently passed certain failure modes.
   For ref == +/-inf, computing (our - ref) / ref produces NaN (when
   our is also inf), and NaN > tol is false, so the check passed even
   when our had wrong sign or was finite. For ref finite and our NaN,
   the same arithmetic produced NaN > tol = false, masking the failure.

   Fix: add explicit special-case handling at the top of the lambda:
     - std::isnan(ref) -> require std::isnan(our) (existing behavior,
       restated using std::isnan for clarity)
     - std::isinf(ref) -> require std::isinf(our) AND matching std::signbit
     - !std::isfinite(our) when ref is finite -> count as failure
     - both finite -> the existing (our-ref)/ref relative-error check

Verified cm_pow PASSes on gcc and clang with the hardened checks (which
shows the implementation has always been correct -- only the tests were
under-strict).

Relates to #765, #770

Co-auth... (continued)

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