stillwater-sc / universal / 25283492920

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Commit Message

feat(qd_cascade): full constexpr support across all operators (#799)

Promotes qd_cascade (quad-double via floatcascade<4>) to fully constexpr
across construction, arithmetic (+, -, *, /, +=, -=, *=, /=), comparison
(==, !=, <, >, <=, >=), unary negation, and conversion-out (operator
int/long/long long/unsigned*/float/double).  Completes the cascade
trio (Epic #723):
  - dd_cascade: #797 (a28f6727)
  - td_cascade: #798 (625032bf)
  - qd_cascade: this PR

Most of the floatcascade<N> EFT primitives were already promoted to
constexpr in #797 (two_sum, fast_two_sum, two_prod, three_sum,
three_sum2, isneg/ispos).  This PR adds the N=4 specializations.

floatcascade.hpp changes:
  - compress_8to4: marked constexpr (pure fast_two_sum chain).
  - renormalize<4> specialization: marked constexpr; std::isinf
    replaced with sw::universal::is_inf_cx (constexpr from #796).
  - **Added constexpr non-template overloads** for floatcascade<4>:
      add_cascades(floatcascade<4>, floatcascade<4>) -> floatcascade<8>
      multiply_cascades(floatcascade<4>, floatcascade<4>) -> floatcascade<4>
    These pick up before the generic templates via overload resolution
    when called with floatcascade<4> args.  add_cascades sorts 8
    elements by magnitude with ternary-abs bubble sort.
    multiply_cascades computes 16 partial products with two_prod, sorts
    the 32-term expansion in a fixed-size array (no std::sort), then
    accumulates with two_sum chains and renormalizes.

qd_cascade_impl.hpp changes:
  - Forward declarations of operator-, operator* promoted to constexpr.
  - Conversion operators int / long / long long / unsigned* / float /
    double: marked constexpr.  Float/double inlined as cascade.to_double().
  - Integer conversions go through new private convert_to_signed_impl /
    convert_to_unsigned_impl templates that collapse the lower three
    limbs (mid_hi + mid_lo + lo) into a single representative double via
    a two_sum chain (sub-ULP error discarded... (continued)

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11 existing lines in 2 files now uncovered.

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