Arcp.Core.md mis-sketches Envelope (record vs class) and Extensions type #62

Category: docs Severity: minor
Location: docs/projects/Arcp.Core.md:19-32

What

Envelope is a sealed record, not a class, and Extensions is IDictionary<string, JsonElement>? { get; init; } (nullable, interface-typed, init-only), not a get-only Dictionary (src/Arcp.Core/Envelope/Envelope.cs:11,62). The sketch misrepresents type, nullability, and mutability.

Evidence

public sealed class Envelope
{
...
    // Unknown top-level fields round-trip here (§15):
    public Dictionary<string, JsonElement> Extensions { get; }
}

Proposed fix

Show public IDictionary<string, JsonElement>? Extensions { get; init; } on a sealed record, and note it may be null.

Acceptance criteria

• The sketch matches Envelope's actual record/property declarations.