stillwater-sc / universal / 24963550987

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feat(lns): full constexpr support across all operators (#776)

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

Promotes lns<nbits, rbits, BlockType> to fully constexpr across all
user-facing operators. Joins integer (#720), posit (#718), cfloat (#719),
bfloat16 (#725), and fixpnt (#721) as a fully constexpr-compliant
Universal type.

The hard prerequisite -- compile-time log2/exp2/pow/sqrt/log/exp -- is
the sw::math::constexpr_math facility that just landed in Epic #763.
With those in place, the lns work is mostly mechanical:

lns_impl.hpp changes:
  - #include <math/constexpr_math.hpp>
  - convert_ieee754: std::log2 -> sw::math::constexpr_math::log2
    (cm only has float/double overloads; long double routes through
    double, lossy at the long-double tail but well below lns precision
    which has at most 64 bits of significand anyway)
  - to_ieee754: std::pow(2, value) -> sw::math::constexpr_math::exp2(value)
    (more direct fit than pow with constant base; same long-double
    routing as above)
  - convert_ieee754 Saturating branch: std::abs -> ternary
    (constexpr-safe, avoids a non-constexpr std::abs call)
  - operator+=, -=, *=, /= and double-overloads: add constexpr
  - operator++, --, postfix ++(int), --(int): add constexpr
  - operator*= / /= Saturating branches: drop `static` from
    `static constexpr ExponentBlockBinary` declarations -- `static
    constexpr` inside a constexpr function is C++23-only (would need
    -std=c++2b) and Universal targets C++20

Add/sub still use the placeholder double-trip arithmetic
(`double(*this) + double(rhs)` -- the `// TODO: native implementation`
comment dates to 2017). Now that both casts are constexpr-callable, the
double-trip works at compile time. The proper Gauss log-add formula
log2(1 + 2^(b-a)) is still a future native-implementation task,
unrelated to constexpr support.

Divide-by-zero throw is gated under LNS_THROW_ARITHMETIC_EXCEPTION;
default (=0) returns NaN encoding, which is ... (continued)

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