[TS PBT] Save campaign artifacts before stdout publication

[CaelmBleidd]

campaign-file could lose a completed, serialized result when stdout failed or the outer deadline interrupted its output: the destination file was written afterward. Save the existing encoded JSON before appending it to stdout, so an output failure leaves a readable artifact. A finally block preserves stdout fallback when the file write fails. The command retains identical file/stdout JSON and encodes it once.

Closes #612. Based on #581 (028d5b0c116502525cdb2329f24bde665807922f); only the command and its focused test change. The caller/JVM deadline-origin gap remains tracked in #613. This change does not explain or resolve every zero-stdout missing-artifact run, and a killed process remains incomplete for primary experimental credit.

Validation:

• The new failing-output regression fails on the unchanged parent with NoSuchFileException.
• All 4 CampaignFileCliTest tests pass after the change. A failing sink reads the saved artifact before its first stdout write, then throws IOException; the command reports the output error and retains the artifact. A separate file-write failure preserves the complete stdout artifact and the original error.
• Strict :usvm-ts-fast-check:detektMain / detektTest and git diff --check pass.

The frozen experimental assembly and measurement artifacts were not changed. No scientific pilot was rerun.