[repo-assist] docs: add missing XML documentation to public API; update README features

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Summary

Adds XML documentation comments (<summary>, <param>, <returns>, <exception>) to every previously undocumented public member, and updates the README features list to be comprehensive.

What was missing

The following public members had no XML documentation:

• Properties: MaxValue, MinValue, Zero, One, Epsilon, NegativeOne
• Constructor: FixedPointNano(long rawValue)
• Static methods: Abs, FractionalPart, IsInteger, FromDecimal, FromRaw, CopySign, Lerp, Clamp, Sign, Parse (both overloads), TryParse (all 4 overloads), Round, Divide(int), Pow, PopulationVariance, SampleVariance, SampleStandardDeviation, Sqrt, Truncate, Frac
• Instance methods: CompareTo(FixedPointNano), Equals(FixedPointNano), Deconstruct, ToDecimal, ToDouble, ToSingle
• Operator: operator *

README was also missing Parse/TryParse and the full math helper list (Clamp, Sign, CopySign, Lerp, Pow).

Changes

• src/FixedPointNano/FixedPointNano.cs — 194 lines of XML docs added
• README.md — features list updated

Test Status

✅ dotnet build — succeeded, 0 warnings
✅ dotnet test — 1426/1426 passed

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Protected Files

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Protected files

• README.md

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