V0 From{Unit} factories throw ArithmeticException for NaN instead of the documented ArgumentException #326

What's wrong

Vector0Guards.EnsureNonNegative and EnsurePositive (Semantics.Quantities/Vector0Guards.cs around lines 34 and 60) test the value with T.Sign(value). For double, float and Half, Sign(NaN) throws ArithmeticException ("Function does not accept floating point Not-a-Number values."), so the documented ArgumentException is never reached.

Repro

Speed<double>.FromMeterPerSecond(double.NaN), Mass<double>.FromKilogram(double.NaN) and Wavelength<double>.FromMeter(double.NaN) all throw ArithmeticException.

Why it matters

Four places say this should be an ArgumentException:

• the guard XML docs;
• every generated factory's XML (for example Speed.g.cs around line 101), which documents only ArgumentException;
• the Quantities docs;
• CLAUDE.md ("Throw ArgumentException for validation failures").

A caller that validates user input by catching ArgumentException gets an unhandled ArithmeticException instead. #285's suggested fix routes scalar * and / through these guards, which would spread the same exception to arithmetic results such as 0 * ∞.

Suggested fix

In both guards, check T.IsNaN(value) first and throw ArgumentException($"Magnitude must be a number; received NaN.", paramName). Then compare with value < T.Zero / value <= T.Zero rather than T.Sign.

Acceptance: add tests for both guards with double.NaN, float.NaN and Half.NaN that assert ArgumentException, plus a generated-factory test such as Speed<double>.FromMeterPerSecond(double.NaN).