stillwater-sc / universal / 24916020162

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Commit Message

feat(internal): promote blocktriple and blocksignificand arithmetic to constexpr (#753)

* feat(internal): promote blocktriple and blocksignificand arithmetic to constexpr

Foundational prerequisite for posit/cfloat/lns/etc. constexpr arithmetic
work tracked under Epic #723. Mirrors PR #716 (blockbinary arithmetic).

Promoted to constexpr:
  blocksignificand::add, sub, mul, div  (the 5 non-constexpr operators
                                         on a class that was already mostly
                                         constexpr-clean)
  blocktriple::add, sub, mul, div       (4 functions; all bodies already
                                         used `if constexpr (_trace_X)` and
                                         delegated only to constexpr-eligible
                                         _significand methods)
  free blockbinary operator==, !=, <, <=, >, >= and the matching friend
  declarations (bit-only operations; trivial promote)

Implementation notes:
- blocksignificand::add's bitsInBlock==64 path uses addcarry, which is
  not constexpr on MSVC. Same std::is_constant_evaluated() dispatch
  pattern as PR #716 for blockbinary: portable carry path in constant-
  evaluated context, intrinsic at runtime. Zero performance impact.
- blocksignificand::sub, mul, div delegate to add and inherit the constexpr
  treatment automatically.
- blocktriple::add/sub/mul/div delegate to _significand methods which are
  now all constexpr. Just needed the keyword.
- blockbinary's free comparison operators are bit-only loops; constexpr-
  eligible by inspection.

Cross-type regression validation (gcc-13 + clang-18, -std=c++20):
  blockbinary: bb_constexpr, bb_api, bb_conversion         PASS
  posit:       posit_api, posit_assignment, posit_conversion,
               posit_addition, posit_subtraction, posit_multiplication,
               posit_division                              PASS
  cfloat:      cfloat_api, cfloat_assignment, cfloat_constexpr  PASS
 ... (continued)

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19 of 29 new or added lines in 3 files covered. (65.52%)

19 existing lines in 3 files now uncovered.

44937 of 53277 relevant lines covered (84.35%)

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