Use byte arrays for closed SOAP hex locals

Replace constructor-initialized method-local SoapHexBinary wrappers only when all references are attributed Value reads or parameterless ToString calls. Capture the same byte-array reference at the original declaration, retain alias mutations, and format with Convert.ToHexString only where ToString was called, preserving its null exception timing. Parsing, writes, escapes, captures/query expressions, nameof and unresolved flows remain for review. Targets .NET 5+.