Serialization.V2: remove [AkkaEnvelopePayload]; a property typed object is the serializer boundary

[Aaronontheweb]

Summary

Part of #8384. Implements Decision 20 of the OpenSpec design record (openspec/changes/messagepack-sourcegen-validation/design.md): [AkkaEnvelopePayload] is removed, and a property typed object is the serializer boundary on its own.

The static type of a property, after generic substitution, selects its encoding:

• object or object?: the envelope frame, {1: serializerId, 2: manifest, 3: bytes}, decoded by whatever serializer owns the payload's runtime type.
• an interface or abstract class with a type-level [AkkaUnion]: the union frame.
• an interface or abstract class with no closed set: AKKASG003, with the hint "Declare a closed member set with [AkkaUnion], or type the property as object."
• a concrete serializable type: inline.

A generic wrapper follows its type argument: Envelope<object> is a boundary, Envelope<AcceptCassette> is inline.

Changes

• AkkaEnvelopePayloadAttribute deleted. XML docs on AkkaSerializer updated.
• Generator: envelope-payload field kind now derives from SpecialType.System_Object on the substituted member type. Attribute detection removed.
• AKKASG035 removed; its conflict cannot occur. The id stays a permanent gap.
• New AKKASG038 (Error): a field-level [AkkaUnion] on an object-typed property is contradictory intent. Message: "Property '{0}' on type '{1}' is typed object, which is always an envelope payload boundary, but carries a field-level [AkkaUnion]. Type the property as the union's base type, or remove the attribute."
• AKKASG003 polymorphic hint text updated as above. Ids, titles, and severities unchanged.
• Retyped boundary fields: Artery SystemMessageEnvelope.Message (ISystemMessage to object), benchmark BenchmarkOuterEnvelope.Inner, and test fixtures; casts added at the consumers that needed them. The envelope nesting-depth guard is untouched.

Why

The attribute only ever added a static type to a boundary field. On dev, eight usages existed: four already object, two interfaces in a DTO and a spec, two concrete envelope types in nesting tests. All are DTO mirrors or fixtures, where retyping costs almost nothing. Same principle as the rest of the design: the closed set comes from the type, and the open case is explicit because the author wrote object.

Compatibility

No wire-format change: emitted code for an envelope field is byte-identical, and no golden-output or wire-snapshot file changed. The attribute never shipped in a release, so no BREAKING_CHANGES_V1.6.md entry. This must land before the first 1.6 beta; after that it would be a breaking change. Akka.Serialization.V2 is not covered by the API approval suite, and the retyped Artery type is internal; the approval tests pass unchanged.

Testing

dotnet test src/core/Akka.Serialization.V2.Tests -c Release: 268 passed, 0 failed (two AKKASG035 tests removed, one AKKASG038 test added). Akka.Remote builds; ArteryControlMessageSerializerSpec 32/32. Benchmarks build.

Follow-up

The user guide (#8514, merged) documents the attribute as it shipped before this PR and lists this removal under planned changes. A docs PR updates the envelope-payloads section once this merges.

[Aaronontheweb]

LGTM