Convert NFloat in CreateChecked, CreateSaturating, CreateTruncating and To() [patch]

[matt-edmondson]

Fixes #138

What was wrong

TryConvertFrom and TryConvertTo handled every built-in numeric type except System.Runtime.InteropServices.NFloat. For NFloat:

• CreateChecked, CreateSaturating and CreateTruncating threw NotSupportedException in both directions.
• To<NFloat>() fell back to NFloat.CreateChecked(BigInteger), which also throws, even for 1.5.

ToPreciseNumber() already accepted NFloat, so the API took NFloat in but refused to give it back.

Change

• From NFloat: routed through TryConvertFromBinaryFloatingPoint, the same path double, float and Half use, with the same NaN and infinity rules. The result matches ToPreciseNumber().
• To NFloat: rounded once, through ToSingle() when NFloat.Size is 4 and ToDouble() otherwise. It is correctly rounded on either platform and overflows to infinity like the other binary floats. To<NFloat>() picks this up because it calls TryConvertTo first.
• To<T>() remarks: NFloat is now listed with the correctly rounded types. The remarks also say that the fallback for any other type needs CreateChecked from BigInteger and double, or it throws NotSupportedException. The issue offered either documenting this or making the fallback work; I documented it.
• Docs: the TryConvertToChecked remarks and the conversions bullet in CLAUDE.md now mention NFloat.

Tests

New tests in PreciseNumberConversionTests:

• FromNFloatMatchesToPreciseNumber: all three modes for 1.5, -0.3048 and 0. For 0.1, Epsilon (denormal), MaxValue and MinValue, CreateChecked agrees with ToPreciseNumber(). Also covers NaN (throws when checked, gives 0 when saturating) and infinity (throws).
• ToNFloatIsCorrectlyRounded: 1.5 and 0.1 in all modes and through To<NFloat>(). Values beyond range give ±infinity, and values below it give 0.
• NFloatRoundTripsThroughPreciseNumber: 1.5, 0.1, Epsilon, MaxValue, MinValue and 1/3 round-trip through all three modes and To<NFloat>().

With the PreciseNumber.Conversions.cs change reverted, all 3 fail. With it, the full suite passes locally on net10.0 (516/516). The library also builds for net7.0 through net10.0.

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Quality Gate passed

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0 New issues
0 Accepted issues

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