stillwater-sc / universal / 30315637420

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feat(fixpnt): add fused fma(a,b,c) with single rounding (#1230)

* feat(fixpnt): add fused fma(a,b,c) with single rounding

fixpnt had no fma. Add a free sw::universal::fma(a,b,c) that forms a*b + c in
a wide exact intermediate and rounds ONCE, matching std::fma's fused contract.

The product a*b is formed exactly as a 2*nbits two's-complement value via urmul2
(the same primitive fixpnt::operator*= uses); c is sign-extended and shifted into
the product's 2*rbits-fractional frame and added exactly; only then is the result
rounded (round-to-nearest-even at rbits, via blockbinary::roundingMode) and
range-limited (Modulo wrap / Saturate clamp). This mirrors operator*='s rounding
model but folds c in before the single rounding, so fma is strictly more accurate
than the two-rounding fixpnt(a*b) + c whenever the product's low-order bits matter.

Test (static/fixpnt/binary/math/fma.cpp) validates against an INDEPENDENT __int128
integer oracle (a,b,c are exact multiples of 2^-rbits, so a*b+c is an exact
rational; the oracle rounds it round-to-nearest-even then applies the range
policy): exhaustive (a,b) for small configs, randomized correctly-rounded checks
across Modulo/Saturate up to fixpnt<32,16>, single-rounding superiority (the naive
two-rounding form disagreed with the exact result in 30/5000 cases; fma matched
the oracle in all), identities, and saturation/overflow corners.

Note: the wide accumulator must be explicitly zeroed -- blockbinary is trivially
constructible, so gcc happened to clear the stack but clang did not, which leaked
into fma(0,b,c). Green on gcc + clang.

Resolves #1191
Relates to #1189

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

* fix(fixpnt): drop __int128 from fma test oracle (MSVC has no __int128)

MSVC does not support __int128 (error C4235), breaking the Windows/MSVC CI build.
Rewrite the fma test's integer oracle in exact int64: fold c in as the integer
part (value = (A*B/2^rbits + C)) instead of mate... (continued)

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