stillwater-sc / universal / 30378198335

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Commit Message

feat(posit1/posito): add a rounded scalar fma overload (#1238)

posit1 and posito's fma returned an UNROUNDED internal::value building block -- a
building block, not a drop-in scalar. Add a public fma overload that returns the
posit/posito type with a SINGLE rounding (exact product-sum via the retained
building block, then one convert()), matching the modern posit fma. The unrounded
variant is retained, renamed to fma_value. Since C++ cannot overload on return type,
the rename is required.

posito latent bug: posito's fma_value decoded its inputs with extract_fraction<>,
which is defined for posit1, not posito -- a type error that was dormant because the
posito fma template was never instantiated. Fixed to use posito's own normalize()
decode path (the same one operator*= uses). (posito's fam has the same dormant
extract_fraction issue; left as-is since it is out of scope for this fma change.)

Tests: static/tapered/posit1/arithmetic/fma_rounded.cpp and
static/tapered/posito/arithmetic/fma.cpp validate the rounded fma against an
independent fused reference (round(std::fmal), exact for small posits): correctly
rounded, single-rounding superiority over the naive two-rounding a*b + c under
cancellation (~29% of random cases differ), and NaR propagation. The existing
posit1 arithmetic/fma.cpp demo still passes (it rounded via assignment). Green on
gcc + clang.

Resolves #1197
Relates to #1189

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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