Envelope missing required event_seq field (§8.1)

[nficano]

Category: spec-conformance Severity: blocker
Location: src/Envelope/Envelope.php:30-66
Spec: ARCP v1.1 §8.1

What

§8.1 mandates each event envelope carry type, session_id, job_id, event_seq, and payload. The Envelope DTO never defines an event_seq property, EnvelopeSerializer/EnvelopeMetadataCodec never reads or writes it.

Evidence

public function __construct(
    public MessageId $id,
    public MessageType $payload,
    public \DateTimeImmutable $timestamp,
    public Priority $priority = Priority::Normal,
    public ?SessionId $sessionId = null,
    public ?JobId $jobId = null,
    public ?StreamId $streamId = null,
    public ?SubscriptionId $subscriptionId = null,
    public ?TraceId $traceId = null,
    ...

Proposed fix

Add an eventSeq property to Envelope, allocate it from a session-scoped monotonic counter when runtime.emit() builds the envelope, and serialize as event_seq in EnvelopeMetadataCodec.

Acceptance criteria

• Every envelope serialized by EnvelopeSerializer carries an integer event_seq; per-session counters are strictly monotonic.