stillwater-sc / universal / 30361765360

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feat(areal): add fma(a,b,c) with faithful rounding + uncertainty-bit semantics (#1233)

areal (faithful float with an uncertainty bit) had no fma. Add a free
sw::universal::fma(a,b,c) that widens to double, forms a*b + c with std::fma, and
faithfully rounds the result once into areal via the value constructor.

Uncertainty-bit semantics (documented in math_functions.hpp): areal's fraction LSB
is the ubit -- 0 when the value is exact, 1 when the stored value is the truncated
representative of the open interval to the adjacent encoding that contains the true
result. fma is therefore a single faithful rounding of a*b + c into areal: exact
results carry ubit=0, inexact results carry ubit=1 with the true value within one
ulp. For configurations whose significand is narrow enough that a product is exact
in double (2*fbits <= 52) the double intermediate carries the correctly-rounded
a*b + c; std::fma supplies the IEEE special cases (inf*0 -> NaN, inf/NaN propagation).

Test (static/range/areal/math/fma.cpp, new areal math target) validates against an
independent long-double reference (computed off the implementation's double/std::fma
path): exactness (exact results -> ubit=0), faithfulness (|value - true| < ulp with
ubit=0 iff exact) over random operands for areal<16,5> and areal<20,6>, and IEEE
special values. Adds the missing math/ glob to the areal test CMakeLists. Green on
gcc + clang.

Resolves #1194
Relates to #1189

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

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