stillwater-sc / universal / 25264031665

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feat(dbns): full constexpr support across all operators (#795)

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

Promotes dbns<nbits, fbbits, BlockType, ...> to fully constexpr across
all user-facing operators. Joins integer (#720), posit (#718),
cfloat (#719), bfloat16 (#725), fixpnt (#721), lns (#776), and
areal (#792) as a fully constexpr-compliant Universal type.

Per Epic #723, the proven playbook (lns PR #776) requires three
mechanical changes plus one structural fix:

dbns_impl.hpp changes:
  - #include <math/constexpr_math.hpp>
  - convert_ieee754: std::log2 -> sw::math::constexpr_math::log2
    with std::is_constant_evaluated dispatch for long double; preserve
    runtime long-double range via std::log2 at runtime, route through
    double at compile time (literal long-double inputs are bounded).
  - convert_ieee754: std::round/std::abs/std::pow -> constexpr
    equivalents (inline ternary for round/abs; cm::exp2/cm::pow inside
    the dead bDebug branch).
  - to_ieee754: ipow() promoted to constexpr (purely arithmetic loop,
    no stdlib calls).
  - operator+= -= *= /= and double-overloads: add CONSTEXPRESSION
    (the +=/-= path goes through double(*this) which is now constexpr).
  - operator++ -- (prefix and postfix): add constexpr.
  - explicit operator int/long/long long/float/double: add CONSTEXPRESSION.
  - explicit operator long double + long double ctor + operator=:
    add CONSTEXPRESSION.
  - dbnsStats event-statistics writes: gated under
    !std::is_constant_evaluated() so static-state mutation only happens
    at runtime (constexpr-safe).
  - assert(best_b >= 0) replaced with comment (assert is not
    constexpr-safe and the invariant is enforced by the search loop).

Pre-existing fix piggybacked: operator<(const dbns&, const dbns&)
contained latent broken code (constructed blockbinary from a raw
BlockType[] array, no such ctor exists). Never instantiated by any
existing test, so the bug went unnoticed. Replaced with a... (continued)

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