vendor-extensions.md writes into the init-only, null-by-default Extensions

[nficano]

Category: docs Severity: major
Location: docs/guides/vendor-extensions.md:33-36

What

Envelope.Extensions is IDictionary<string,JsonElement>? with an init-only setter (src/Arcp.Core/Envelope/Envelope.cs:62). On a freshly constructed Envelope it is null, so the indexer assignment throws NullReferenceException at runtime; it also cannot be reassigned after construction.

Evidence

var env = new Envelope { /* ... */ };
env.Extensions["x-vendor.acme.priority"] = JsonSerializer.SerializeToElement("high");
await transport.SendAsync(env, ct);

Proposed fix

Set Extensions in the object initializer, e.g. new Envelope { ..., Extensions = new Dictionary<string, JsonElement> { ["x-vendor.acme.priority"] = ... } }.

Acceptance criteria

• The example populates Extensions via the init-only property without dereferencing null.

[nficano]

Fixed in #77 (merged).