v1.1 session.resume is absent; legacy resume is not dispatched (§6.3)

[nficano]

Category: spec-conformance Severity: blocker
Location: crates/arcp-core/src/messages/control.rs:96-103
Spec: ARCP v1.1 §6.3

What

The SDK models only the legacy resume payload, and run_connection has no dispatch arm for MessageType::Resume or a v1.1 session.resume. There is no resume-token rotation, no last_event_seq replay, and no RESUME_WINDOW_EXPIRED response path.

Evidence

/// Payload for the SDK's legacy `resume` envelope.
///
/// The v1.1 resume contract is `session.resume` (ARCP v1.1 §6.3) carrying
/// `last_event_seq` and `resume_token`; this older shape
/// (`after_message_id` / `checkpoint_id`) is retained for backward
/// compatibility with v1.0 peers.
#[derive(Debug, Clone, Default, PartialEq, Eq, Serialize, Deserialize)]
pub struct ResumePayload {

Proposed fix

Add the v1.1 session.resume message shape, issue and rotate resume tokens on welcome/accepted, replay buffered events by last_event_seq, and return RESUME_WINDOW_EXPIRED when coverage is gone.

Acceptance criteria

• A client can reconnect with (resume_token, last_event_seq) and receive only later buffered events; stale or uncovered sequence requests return RESUME_WINDOW_EXPIRED.