stillwater-sc / universal / 30371151083

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feat(takum): add fma(a,b,c) (#1235)

takum (linear takum encoding, epic #592) had no fma. Add a free sw::universal::fma
that widens to double, forms a*b + c with std::fma, and rounds the result once into
takum via the value constructor. A takum's significand precision is well under
double's 53 bits for practical configurations, so the double intermediate carries
the correctly-rounded a*b + c and the double -> takum rounding is the single rounding
that determines the result. takum's NaR state absorbs the IEEE specials (inf*0 -> NaN
-> NaR, any inf/NaN operand -> NaR).

Test (static/tapered/takum/math/fma.cpp) validates against an independent long-double
reference (off the implementation's double/std::fma path): exactness for
exactly-representable results, correctly-rounded agreement with takum(reference) over
random operands for takum<32,5> and takum<24,4>, and NaR handling for inf/NaN.
Green on gcc + clang.

Resolves #1195
Relates to #1189, #592

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

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