stillwater-sc / universal / 25268392612

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feat(dd): full constexpr support across all operators (#796)

* feat(dd): full constexpr support across arithmetic and comparison operators

Promotes dd (double-double) to fully constexpr across construction,
arithmetic (+, -, *, /, +=, -=, *=, /=), comparison (==, !=, <, >, <=,
>=), unary negation, and conversion-out (operator int/long/long long/
unsigned int/unsigned long/unsigned long long/float/double, plus
long double when LONG_DOUBLE_SUPPORT is enabled). Joins integer (#720),
posit (#718), cfloat (#719), bfloat16 (#725), fixpnt (#721), lns (#776),
areal (#792), and dbns (#795) as a fully constexpr-compliant Universal
type.

The structural blocker for dd was that its compound arithmetic relies on
error-free transformations (two_sum, two_prod, three_sum, split, renorm,
quick_two_sum, two_diff, two_sqr) defined in error_free_ops.hpp, all of
which used `volatile double&` out-parameters and `std::isfinite` checks.
Volatile-qualified glvalue access and pre-C++23 std::isfinite are not
permitted in constant evaluation; both are however load-bearing for
runtime IEEE-754 correctness on aggressive compilers (per the
floatcascade volatile-hardening doc).

error_free_ops.hpp changes:
  - Removed `volatile` from the output parameter signatures (the volatile
    on intermediates is what protects the EFT against compiler reordering;
    the output parameter is just a normal store).
  - Each EFT primitive now dispatches via std::is_constant_evaluated():
    the runtime branch keeps its volatile intermediates and std::isfinite,
    the constexpr branch uses pure double arithmetic with a new
    is_finite_cx helper that reads NaN/Inf via well-defined comparison.
  - Added is_finite_cx() and is_inf_cx() constexpr helpers (analogues of
    std::isfinite/std::isinf for C++20 constant evaluation).
  - split() in its constexpr branch substitutes std::ldexp(x, +/-(BITS+1))
    with a multiplication by 2^(BITS+1) (a constexpr-clean equivalent for
    the SPLIT_THRESHOLD ... (continued)

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9 existing lines in 2 files now uncovered.

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