Add generic NumberSource<T> and NullableNumberSource<T> with min/max

[holonsoftFounder]

Summary

• NumberSource<T> / NullableNumberSource<T> for every built-in numeric type (T : INumber<T>, IMinMaxValue<T>): integers up to 128 bit incl. Int128, UInt128, nint, char; float, double, Half, decimal
• integers are drawn uniformly from the inclusive range: width computed in modular UInt128 arithmetic (exact for signed and unsigned up to 128 bit), Random.NextInt64 for widths below long.MaxValue, 16 random bytes with rejection sampling above that
• floating point and decimal are interpolated as min * (1 - s) + max * s, which cannot overflow even for the whole range of the type (the existing DoubleSource() default produces infinity)
• bounds are validated on construction and on SetMinMax/SetMin/SetMax: max below min, NaN and infinity throw ArgumentOutOfRangeException, the old range survives a failed change
• NullableNumberSource<T> takes an explicit null creation threshold
• existing IntegerSource, DecimalSource and friends untouched, defaults untouched, no stable sequence changed
• README section and PackageReleaseNotes

Test plan

• 25 new tests: range per type, whole-type ranges, both ends of short ranges, every byte value, rough uniformity for small ranges and for a range wider than long, min == max, validation, setters, nullable threshold, stable sequences, integration via Use<>(ctor args), Use<>(lambda), field, generation time Source(...)
• dotnet build -c Release zero warnings
• 707 tests green on net8, net9 and net10 (Windows)

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