MessagePack.Annotations (migration shim)

In MessagePack v4 the attributes ([MessagePackObject], [Key], [IgnoreMember], [SerializationConstructor], [MessagePackFormatter]) and IMessagePackSerializationCallbackReceiver live in the MessagePack package itself. The source generator ships there too and runs on the assembly that declares the annotated types, so that assembly has to reference MessagePack anyway. An annotations-only reference is no longer a supported setup.

This package exists so that projects coming from v3 keep working:

• The surviving types are type-forwarded to MessagePack, so using MessagePack; code compiles unchanged and the attribute types a pre-built v3 assembly references resolve (its formatter code, bound to v3's reader and writer, still cannot run on v4).
• The package depends on MessagePack, so bumping its version brings the v4 runtime and source generator in.
• Referencing the 4.x version keeps MessagePack.Annotations 3.x from sitting next to MessagePack 4.x, which would define every attribute twice (CS0433). NuGet does not do this on its own: MessagePack does not depend on this package (the dependency runs the other way), so a transitive 3.x reference from a v3-built library stays at 3.x until the consuming project adds an explicit PackageReference to MessagePack.Annotations 4.x.
• The types v4 renamed or deleted ([Union], [MessagePackKnownFormatter], [MessagePackAssumedFormattable], [ExcludeFormatterFromSourceGeneratedResolver]) still exist here in their v3 shape, marked [Obsolete] with error: true. The compiler error text tells you the replacement.

Once your code compiles, remove this package and reference MessagePack directly.