Serialization.V2: object elements inside collections are envelope-payload boundaries (stacked on #8521)

[Aaronontheweb]

Summary

Part of #8384. Extends Decision 20 (OpenSpec design record) to collection elements: a supported collection whose element type is object or object? treats each element as an envelope-payload boundary, the same frame a property typed object gets since #8518. Closes the limitation that #8520 documented in the guide.

Stacked on #8521 (S1); the diff is one commit. It is independent of #8522 (S2) and rebases onto whichever lands first.

What changes

• Extraction. An element typed object maps to the envelope-payload kind, nullable-aware, for every collection shape the generator supports: arrays, lists, read-only lists and collections, sets, immutable collections, and dictionary values. All of them funnel through one element-mapping routine, so there is no per-shape special case.
• Emission. The three element emitters gain an envelope case that calls the same runtime helpers the property-level case calls, with the same nil framing for a nullable element and the same UnknownSize propagation for size. Every other element kind is byte-for-byte untouched.
• Dictionary keys typed object stay rejected with AKKASG003, and the reason is in a code comment: a null key crashes a dictionary at run time, and an envelope payload is a fresh instance on read, so a round-tripped key has no stable identity for lookup.

Tests and snapshots

• ObjectElementSpec, eight facts: round trips for List<object>, object[], Dictionary<string, object>, and ImmutableList<object> holding a message from the same serializer, a string, and an int; a nullable element holding null; SizeHint reporting UnknownSize when an element's serializer cannot size it; the nesting-depth guard reached through a collection element; and the AKKASG003 fact for Dictionary<object, string>.
• Golden output gains one message with a List<object> field. The diff against the verified file is purely additive; every pre-existing line is unchanged. The model snapshot from S1 gains the same message.
• A new wire snapshot, envelope-list-element-string-and-message, pins the bytes for a list holding a string and a message.
• dotnet test src/core/Akka.Serialization.V2.Tests -c Release: 287 passed. Generator builds with warnings as errors. Akka.Remote builds.

OpenSpec

Scenario "Object elements inside a collection are boundaries" added under the envelope-boundary requirement in the source-generator spec; task 5.17 added.

Follow-up

The guide's supported-types table and limitations section list this as unsupported since #8520. A one-line docs change lands after this merges.

[Aaronontheweb]

Closing: this change is being reopened as part of a native GitHub stack with all branches in akkadotnet/akka.net (stacks cannot span a fork). Same commits, same content; the new PR will link back here.