Serialization.V2: pin cross-assembly generator behavior, fix AKKASG023 mislabel, add fix-it hints

openspec/changes/messagepack-sourcegen-validation/design.md

@@ -196,7 +196,7 @@ Two structural shapes were added after the decisions above, both following the c
 
 **Closed generic registrations.** A Roslyn source generator is syntax-driven and cannot reify an open generic type, so a generic `[AkkaSerializable]` type (e.g. `Wrapper<T>`) is never itself serialized — the open definition exists only to host the `[AkkaField]` schema its closed constructions share. `[AkkaSerializable<Wrapper<IOrder>>(Manifest = ...)]`, applied to the `[AkkaSerializer]` class, registers one closed construction; each registration then behaves exactly like an ordinary top-level message, with its own manifest and dispatch arm, and its generic fields resolved against the concrete type arguments. A generic definition that implements the serializer's protocol interface with no registered closed construction fails compilation (`AKKASG022`), and a field typed as an unregistered closed construction fails compilation (`AKKASG023`) — the source-generation analog of System.Text.Json rejecting an unbound generic in `[JsonSerializable]`.
 
-Both features are scoped to the current compilation, the same rule every other generator lookup follows (see the cross-assembly discussion in Decision 8): a union member type or a closed-generic registration declared only in a referenced assembly is invisible to this generator.
+Scope differs between the two features, and `CrossAssemblyBaselineSpec` pins the current behavior. A union member type declared only in a referenced assembly is invisible to this generator. It fails AKKASG015 even when it carries `[AkkaSerializable]` and a manifest in that assembly. A closed-generic registration is not scoped that way. `[AkkaSerializable<T>]` resolves the generic definition through symbol APIs, not through the per-compilation syntax registry that backs the union check. So a closed construction over a generic definition that lives only in a referenced assembly (`Wrapper<T>` in A, `[AkkaSerializable<Wrapper<int>>]` on B's serializer) compiles and generates correctly today. What fails across assemblies is a non-generic `[AkkaSerializable]` type used as a nested field, and a union member declared in a referenced assembly. Before this branch the nested-field case was misreported as AKKASG023 (unregistered closed generic). It now reports AKKASG007, and both diagnostics name the type, its assembly, and the fixes that work today.
 
 ### 14. Always-Explicit Serializer Identity