Respect NaN-safe mode in fma and muladd

[devmotion]

Fixes #860.

With NaN-safe mode enabled, fma and muladd with three Dual arguments, or with Dual-Real-Dual arguments, computed the partials with plain fma/muladd. So Inf * 0 produced NaN partials, unlike the equivalent x * y + z.

The partials are now computed with _fma_partial and _muladd_partial. They are defined next to _mul_partial and _div_partial, following the same pattern. In NaN-safe mode, a zero partial leaves the accumulated partial unchanged. With NaN-safe mode disabled, they are plain fma/muladd. The only other change is that in the Dual-Real-Dual case the two factors are swapped. For floating-point numbers this gives the same result; for nested Duals it can change the last bit of the partials.

The new tests check that fma(x, y, z) and muladd(x, y, z) are isequal to x * y + z in both modes, for the examples in #860.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.29%. Comparing base (a3c0f4f) to head (f98da3c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #861      +/-   ##
==========================================
+ Coverage   91.23%   91.29%   +0.06%     
==========================================
  Files          11       11              
  Lines        1072     1080       +8     
==========================================
+ Hits          978      986       +8     
  Misses         94       94              

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