Add UTF-8 Parse/TryParse/TryFormat polyfills for BigInteger and Complex

[SimonCropp]

Polyfills the UTF-8 members that .NET 11 adds to BigInteger and Complex (dotnet/runtime#117745, from the Performance Improvements in .NET 11 post). Signatures were checked against the 11.0 RC1 ref pack, and the 7.0/8.0/9.0/10.0 ref packs were checked for when each member first appeared.

BigInteger (all targets below net11, needs FeatureMemory)

• Parse(ReadOnlySpan<byte>, NumberStyles = Integer, IFormatProvider? = null)
• Parse(ReadOnlySpan<byte>, IFormatProvider?)
• TryParse(ReadOnlySpan<byte>, out BigInteger)
• TryParse(ReadOnlySpan<byte>, NumberStyles, IFormatProvider?, out BigInteger)
• TryParse(ReadOnlySpan<byte>, IFormatProvider?, out BigInteger)
• TryFormat(Span<byte>, out int, ReadOnlySpan<char> = default, IFormatProvider? = null)

Complex (net7.0–net10.0)

• TryParse(ReadOnlySpan<byte>, NumberStyles, IFormatProvider?, out Complex)
• TryParse(ReadOnlySpan<byte>, IFormatProvider?, out Complex)

Below net7, Complex has no string Parse to delegate to. The UTF-8 TryFormat already exists from net8.

Complex.Parse(ReadOnlySpan<byte>, ...) is not polyfilled. Static extension members compile to plain static methods on Polyfill, so it would collide with BigInteger.Parse(ReadOnlySpan<byte>, ...) (CS0111). This matches how IPAddress.Parse is skipped next to Guid.Parse, and is documented with a //Note:.

All members decode the UTF-8 bytes to a string and delegate to the existing overloads. Results match the BCL, but they allocate a string.

New FeatureNumerics constant

On .NET Framework, BigInteger/Complex live in System.Numerics.dll. SDK-style projects reference it implicitly, but old-style projects may not. Polyfill.targets now sets FeatureNumerics when System.Numerics or System.Runtime.Numerics is in @(ReferencePath), and always sets it for netcoreapp, net5+ and netstandard. It is registered in Identifiers.sharedIdentifiers, BuildApiTest and SplitterTests, and listed in CLAUDE.md.

Verification

• ApiBuilderTests (Debug) regenerated Split and the API list: 71/71 pass
• Tests, Consume, ConsumeNoRefs and ConsumeClassicReferences build across all targets
• New tests pass on net48, net8.0 and net11.0 (BCL). netcoreapp3.1 was not run locally because that runtime isn't installed