Preserve runtime-package semantic outputs in workload normalization

[chubes4]

The runtime workload normalizer dropped semantic top-level result fields from single-result runtime package output.

Observed shape:

{ "success": true, "concept_packet": { ... } }

Expected concept_packet to survive in normalized workload outputs. Actual normalized output had outputs: {}.

Acceptance criteria:

• Single-result workload normalization preserves non-control top-level fields as semantic outputs.
• Explicit typed_artifacts and structured_artifacts arrays are surfaced when returned by runtime packages.
• WP Codebox does not infer typed artifacts from arbitrary semantic outputs unless explicitly returned.
• Tests cover semantic outputs plus explicit typed/structured artifact passthrough.