`session.jobs` entries never populate `last_event_seq` (§6.6)

[nficano]

Category: spec-conformance Severity: minor
Location: src/Arcp.Runtime/JobManager.Listing.cs:53-63
Spec: ARCP v1.1 §6.6

What

The session.jobs response shape in §6.6 includes "last_event_seq" per job (so a dashboard can decide where to subscribe from). JobListEntry defines the property, but ToListEntry never sets it, so it serializes as null for every listed job. Clients have no way to learn a job's current sequence position from the listing.

Evidence

private static JobListEntry ToListEntry(Job j) => new()
{
    JobId = j.JobId.Value,
    Agent = j.Agent.ToString(),
    Status = MapStatus(j.Status),
    Lease = j.Lease,
    ParentJobId = j.ParentJobId,
    CreatedAt = j.CreatedAt,
    TraceId = j.TraceId?.Value,
    // LastEventSeq is never assigned -> always null
};

The job already tracks a per-job event buffer; the high-water mark is available (Job.SnapshotEventHistory().Last().EventSeq, or a cheap counter).

Proposed fix

Expose the job's highest emitted event_seq (e.g. a counter updated in Job.BufferEvent) and set LastEventSeq = j.LastEmittedSeq in ToListEntry.

Acceptance criteria

• A running job that has emitted events reports a non-null, monotonic last_event_seq in session.list_jobs.