Resume modeled inside session.hello rather than session.resume (§6.3)

[nficano]

Category: spec-conformance Severity: minor
Location: src/Arcp.Core/Messages.fs:12-29
Spec: ARCP v1.1 §6.3

What

Spec §6.3 describes resume as a distinct message: 'A client reconnecting after a transport drop presents last_event_seq and its most recent resume_token in session.resume.' The SDK piggybacks resume on session.hello.payload.resume, which is not the wire shape the spec defines. Listed as minor because §6.3's wire example is brief; pair this with f-0041 (no runtime handling at all).

Evidence

/// Payload of `session.hello.payload.resume` (spec §6.3).

type ResumeRequest =
    {
        SessionId: string
        ResumeToken: string
        LastEventSeq: int64
    }

/// `session.hello` payload (spec §6.1, §6.2).

type SessionHelloPayload =
    {
        Client: ClientIdentity
        Auth: AuthPayload
        Capabilities: HelloCapabilities
        Resume: ResumeRequest option
    }

Proposed fix

Introduce a Message.SessionResume/session.resume case with its own payload and remove the Resume field from SessionHelloPayload.

Acceptance criteria

• Resume requests travel as session.resume, not as a nested field inside session.hello.