.NET breaking changes: the contract is wider than the file

Use before changing .NET/C# code whose callers you do not control: a NuGet library, or a repo with PublicAPI.Shipped/Unshipped.txt, ApiCompat, multiple TargetFrameworks, InternalsVisibleTo, or generated (.g.cs) partials. Green builds miss silent breaks from optional parameters, parameter names, sealing, const values, nullability, and #if. USE FOR: "without breaking existing callers"; adding a public method, overload, optional parameter, or interface member to a shipped library; renaming a public method or an internal helper a friend assembly uses; consolidating duplicated logic; changing a signature, nullability, default, or attribute; adding a target framework; RS0016/RS0017 or ApiCompat errors; "is this PR breaking?". For a rename, extract, or merge, also invoke csharp-refactoring. DO NOT USE FOR: an executable whose callers all live in one project; renaming a local variable or private member, even in a shipped library; formatting; framework/SDK/NuGet upgrades (use dotnet-upgrade).