stillwater-sc / universal / 24942394888

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Commit Message

feat(fixpnt): full constexpr arithmetic, comparison, shifts, conversions (#761)

* feat(fixpnt): full constexpr arithmetic, comparison, shifts, conversions

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

Construction from native int and from SpecificValue was already
constexpr. This PR completes the surface using the blockbinary mul/div
constexpr foundation that just landed in #759/PR #760.

fixpnt_impl.hpp changes:
  - to_signed, to_unsigned, to_native: constexpr (TargetFloat path uses
    only multiplies and bit-test, constexpr-clean per C++20)
  - bits() accessor: constexpr (required by Saturating-arithmetic
    operators that call maxpos.bits())
  - Cross-template copy ctor and assignment, move assignment: constexpr
  - All compound assignments (+=, -=, *=, /=, %=, <<=, >>=): constexpr
  - operator/= std::cerr div-by-zero diagnostic gated behind
    !is_constant_evaluated; defined-behavior return paths preserved
    (Modulo: setzero; Saturate: clamp to maxpos/maxneg by sign)
  - All 114 free comparison operators (fixpnt-fixpnt + ~108 mixed
    overloads with int/long/long long/unsigned/float/double/long double
    on both sides): constexpr
  - All 99 free arithmetic operators (binary + and shifts + 60 mixed
    overloads): constexpr
  - abs, onesComplement, twosComplement free helpers: constexpr (with
    matching forward-decl updates in fixpnt_fwd.hpp)
  - friend bool operator==/< declarations match constexpr signatures

fixpnt_fwd.hpp changes:
  - abs, onesComplement, twosComplement forward declarations gain
    constexpr keyword to match implementations

static/fixpnt/binary/api/constexpr.cpp:
  - Added "constexpr arithmetic + comparison (issue #721)" section after
    the existing pi-printing tests. Covers:
      - Acceptance form: Q16.16 2.5 * 1.5 = 3.75 raw bits 0x00... (continued)

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109 of 112 new or added lines in 1 file covered. (97.32%)

11 existing lines in 2 files now uncovered.

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