Add preview/apply document edit workflow

[dsghi]

Summary

• add a two-step document editing flow with PreviewEditDocument and ApplyEditDocument
• back apply operations with a hash-guarded conditional replace path and shared content hashing helpers
• cover the new edit workflow with focused unit tests and add a repo-local NuGet.Config for reliable restore in this environment

Testing

• dotnet test devbrain.slnx /p:RestoreConfigFile=NuGet.Config