stillwater-sc / universal / 28183136515

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feat(efloat): implement IEEE-754 correct rounding and add rounding regression tests (#1104)

* feat(efloat): implement IEEE-754 correct rounding and add rounding regression tests (#1091)

Define RoundingMode enum class and global thread-local efloat_rounding_mode
in efloat_impl.hpp.

Implement static constexpr round_limbs() to evaluate rounding decisions (guard,
LSB, sticky, and sticky_remainder) across all four standard rounding modes.

Integrate round_limbs() cleanly inside the public normalize() method, ensuring
any operation resulting in a size greater than nlimbs automatically rounds and
re-normalizes itself. This dramatically simplifies operator+= and operator*=.

Update divide_limbs() to return a remainder_non_zero flag and integrate rounding
directly inside operator/=. Make normalize() a public method to allow test
harnesses to manually normalize/round custom limb patterns.

Create rounding.cpp to test all four rounding modes (nearest ties-to-even,
to-zero, toward positive, toward negative) under precise, microscopic LSB-level
truncations (rounding size-2 vectors to size-1 efloat specializations).

Resolves #1091
Relates to Epic #1101, PR #1103

* refactor(efloat): generalize RoundingMode to sw/universal/behavior/rounding.hpp

Expose RoundingMode as a standard common enum class inside the
sw::universal namespace under Option A (<universal/behavior/rounding.hpp>).

This makes standard IEEE-754 rounding modes globally available to all number
systems in the Universal Library (e.g., lns, cfloat, and fixpnt), while
efloat_impl.hpp includes the new header cleanly and retains its thread_local
efloat_rounding_mode variable.

Relates to #1091, PR #1104

* fix(efloat): resolve CodeRabbit functional correctness comments

1. Fix division rounding sign tracking: pass result_sign (computed as (_sign != rhs._sign))
   to round_limbs in operator/= rather than the pre-operation dividend's _sign, which
   resolves directional division rounding sign mismatches.
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