stillwater-sc / universal / 25280163148

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Commit Message

feat(dd_cascade): full constexpr support across all operators (#797)

* feat(dd_cascade): full constexpr support across all operators

Promotes dd_cascade (double-double via floatcascade<2>) to fully
constexpr across construction, arithmetic (+, -, *, /, +=, -=, *=, /=),
comparison (==, !=, <, >, <=, >=), unary negation, and conversion-out
(operator int/long/long long/unsigned*/float/double).  Joins integer
(#720), posit (#718), cfloat (#719), bfloat16 (#725), fixpnt (#721),
lns (#776), areal (#792), dbns (#795), and dd (#796) as a fully
constexpr-compliant Universal type.

Per Epic #723.  This issue's structural challenge was that dd_cascade
arithmetic routes through floatcascade<2>'s expansion_ops layer, whose
generic add_cascades<N> and multiply_cascades<N> templates use
std::sort and std::vector (std::sort is not constexpr in C++20 -- it
becomes constexpr in C++26).  The plan: keep the generic templates
intact (they're still consumed by td_cascade and qd_cascade) and add
non-template constexpr overloads for floatcascade<2>.

floatcascade.hpp changes (expansion_ops namespace):
  - two_sum, fast_two_sum: is_constant_evaluated() dispatch.  Runtime
    branch retains the volatile intermediates (load-bearing for
    aggressive-optimization correctness per
    floatcascade_volatile_hardening.md).  Constexpr branch drops
    volatile (constexpr arithmetic is exact under IEEE-754 already).
  - two_prod: same dispatch.  Constexpr branch uses split-based product
    (FMA-free, since std::fma is not constexpr in C++20); runtime
    keeps std::fma.
  - three_sum, three_sum2: marked constexpr (now-constexpr two_sum).
  - compress_4to2: marked constexpr (pure fast_two_sum chain).
  - renormalize<2> specialization: marked constexpr; std::isinf
    replaced with sw::universal::is_inf_cx (constexpr from #796).
  - **Added constexpr non-template overloads** for floatcascade<2>:
      add_cascades(floatcascade<2>, floatcascade<2>) -> floatcascade<4>
      multiply_c... (continued)

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15 existing lines in 3 files now uncovered.

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