Inline-result/result_chunk mixing is only guarded for `.value`; `.ref`/`.empty` after streaming emit job.result without result_id (§8.4)

[nficano]

Category: spec-conformance Severity: minor
Location: Sources/ARCP/Runtime/JobManager.swift:658-680
Spec: ARCP v1.1 §8.4

What

§8.4: implementations MUST NOT mix inline result and result_chunk in one job, and once any chunk is emitted the terminating job.result MUST carry result_id. The mixing guard only covers .value; a handler that emits chunks then returns .ref or .empty produces a terminal job.completed with an inline ref / no result_id, violating both rules.

Evidence

private func toCompleted(jobId: JobId, _ output: ToolOutput) throws -> JobCompletedPayload {
    let streamedResultId = jobs[jobId]?.streamedResultId
    switch output {
    case .value(let value):
        if let streamedResultId { throw ARCPError.invalidArgument(...) }
        return JobCompletedPayload(result: value)
    case .ref(let ref): return JobCompletedPayload(resultRef: ref)
    case .empty: return JobCompletedPayload()
    ...

Proposed fix

When streamedResultId is set, reject .ref (mixing) and treat .empty as a streamed completion that must carry the streamed result_id rather than an empty payload.

Acceptance criteria

• A job that emitted result_chunk(s) and then returns ref/empty either errors (mixing) or produces a job.completed carrying the streamed result_id.