feat: add by-ref struct formatter dispatch and codefix stability - #2251

Summary

• add by-ref serializer/formatter dispatch for struct types with compatibility fallback to by-value interfaces
• route DynamicObjectResolver and source-generated member serialization through by-ref dispatch, with nullable struct fallback fix
• stabilize analyzer codefix output by preserving original line endings and fixing modifier trivia handling
• remove temporary design/plan docs from this branch before PR

Test Plan

• dotnet test tests/MessagePack.SourceGenerator.Tests/MessagePack.SourceGenerator.Tests.csproj -v minimal
• dotnet test MessagePack.sln -v minimal