[TS PBT] Save campaign-file artifacts before stdout publication #612

campaign-file encodes the completed campaign, writes the entire JSON to stdout, and only then saves the requested artifact file. A slow, failed or interrupted stdout operation can therefore discard a result that the runner has already produced and serialized.

The order is present in the minimal public parent #581 (028d5b0c116502525cdb2329f24bde665807922f), in CampaignFileRequest.kt:57-59.

Observed in the frozen local class pilot: multi-algorithms-singly-list-wide / 16061113 / SYMBOLIC_ONLY retained exactly 65536 bytes of campaign JSON on stdout, but no campaign.json. The prefix includes stop=DEADLINE, stoppedAt=CONFIRMATION, elapsedMillis=19611, and Native verification deadline reached; 99 inputs remain unchecked. The outer process guard then returned exit -9. The runner and JSON encoding had completed; stdout publication preceded the missing file write. The retained stdout SHA-256 is c60098bca32dc8ed3bc91816afd5213d5e0a8bab3fa27e7b150da76cc67eed46.

Acceptance:

• Save the requested destination before the first stdout append.
• Keep one JSON encoding and the existing identical stdout/file JSON contract.
• Preserve complete stdout fallback and the original I/O error when the file write itself fails.
• A meaningful regression uses an output sink that checks that a readable artifact already exists on its first append, then fails with an I/O error. The failure must leave the artifact intact.
• Do not rerun the property or add a writer/protocol framework.

This is a narrow result-publication fix under #354. It does not resolve the separate caller/JVM deadline-origin mismatch (#613) or establish the internal cause of other zero-stdout missing-artifact runs. A killed process remains incomplete for primary experimental credit even if its artifact can be retained.