stillwater-sc / universal / 25281498813

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Commit Message

feat(td_cascade): full constexpr support across all operators (#798)

Promotes td_cascade (triple-double via floatcascade<3>) to fully
constexpr across construction, arithmetic (+, -, *, /, +=, -=, *=, /=),
comparison (==, !=, <, >, <=, >=), unary negation, and conversion-out
(operator int/long/long long/unsigned*/float/double).  Continues the
Epic #723 cascade trio: dd_cascade (#797), td_cascade (this), and
qd_cascade (next, #743).

Most of the floatcascade<N> EFT primitives were already promoted to
constexpr in the dd_cascade work (#797): two_sum, fast_two_sum,
two_prod, three_sum, three_sum2, isneg/ispos.  This PR adds the N=3
specializations needed for td_cascade.

floatcascade.hpp changes:
  - compress_6to3: marked constexpr (pure fast_two_sum chain).
  - renormalize<3> specialization: marked constexpr; std::isinf
    replaced with sw::universal::is_inf_cx (constexpr from #796).
  - **Added constexpr non-template overloads** for floatcascade<3>:
      add_cascades(floatcascade<3>, floatcascade<3>) -> floatcascade<6>
      multiply_cascades(floatcascade<3>, floatcascade<3>) -> floatcascade<3>
    These pick up before the generic templates via overload resolution
    when called with floatcascade<3> args.  add_cascades sorts 6
    elements by magnitude (bubble sort with ternary abs) then
    accumulates smallest-to-largest with two_sum.  multiply_cascades
    computes all 9 partial products with two_prod, sorts the 18-term
    expansion by magnitude (constexpr-clean fixed-size array sort,
    no std::sort), and accumulates with two_sum chains into a
    3-component result before final renormalize.

td_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 template... (continued)

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93 of 95 new or added lines in 2 files covered. (97.89%)

6 existing lines in 2 files now uncovered.

45861 of 54477 relevant lines covered (84.18%)

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