stillwater-sc / universal / 25386566044

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feat(takum): full constexpr support across all operators (#817)

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

Promote linear takum encoding to fully constexpr across construction,
assignment, arithmetic (+= -= *= /=), comparison (== != < <= > >=),
unary negation, increment / decrement, and IEEE-754 conversion paths.

Key changes in takum_impl.hpp:

* convert_ieee754: replace std::frexp with bit-extraction.  For an
  IEEE 754 normal v = (1 + rawFrac/2^fbits) * 2^(rawExp - bias) the
  takum decomposition is c = rawExp - bias and m_real = rawFrac/2^fbits
  directly.  Subnormals normalize via a bounded leading-zero shift.
  Long double routes through double's encoding when extractFields uses
  LONG_DOUBLE_DOWNCAST.

* to_ieee754: replace std::exp2(c) with sw::math::constexpr_math::exp2,
  which is exact at integer arguments via direct IEEE 754 bit
  construction in detail::pow2.

* All in-class member operators (=, +=, -=, *=, /=, ++, --, unary -,
  out-conversion to int/long/long long/float/double/long double) marked
  CONSTEXPRESSION (constexpr when sw::bit_cast is constexpr) or
  constexpr (pure bit-math).

* Friend comparison operators (==, !=, <, <=, >, >=) and binary
  arithmetic operators (+, -, *, /) on takum<>: marked constexpr /
  CONSTEXPRESSION as appropriate.  Comparisons reduce to signed integer
  comparison on the raw two's-complement bits and stay constexpr.

* characteristic() / scale() / find_dr() promoted to constexpr.

* operator/=: throw guarded under !std::is_constant_evaluated() so the
  setnar fallback branch is reachable at constant evaluation when
  TAKUM_THROW_ARITHMETIC_EXCEPTION is enabled.

New static/tapered/takum/api/constexpr.cpp covers:

* Static accessor invariants (nbits, rbits, overhead, dr_bits, max_r,
  maxCharBits)
* SpecificValue construction (zero / maxpos / maxneg / nar)
* Round-trip conversion: int -> takum -> double, float -> takum ->
  double, double -> takum -> double, NaN -> NaR
* Arithmetic a... (continued)

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