Credit initial .NET client authorship and correct package docs

[Josh (joshmouch)]

Summary

• credit Joshua Mouch for contributing the initial first-party .NET client and handwritten C# reducers in PR Add a first-party .NET client #206
• distinguish those handwritten reducers from the separately generated wire and model sources
• correct the contributor and release documentation to list the two packages the .NET client actually ships

Validation

• npm run verify:changelog
• npm run verify:change-fragments
• git diff --check

This is a documentation-only correction, so it does not add a changelog fragment.