Keep Polymarket uploader stdout machine-readable

[proerror77]

Change contract

Prevent child ossutil progress/success text from contaminating the Rust polymarket-raw-ops upload JSON stdout contract, while preserving all existing fail-closed upload and readback verification.

Failure evidence

Formal Gate t-jpn6s489bspalfk completed the 3600s observation plus 601s mature tail, then failed at jq with Invalid numeric literal. The same run's upload-status.json records uploaded_segments=1, canonical_uploaded_segments=1, failed_segments=[], and the exact OSS triplet exists. A direct ECS probe confirms aliyun ossutil cp writes a human success summary to stdout even with --quiet or --no-progress.

Acceptance evidence

• A focused red-capable test proves child stdout is suppressed by the shared OSS command runner.
• The focused test fails before and passes after the fix.
• Existing Polymarket upload tests, collector test target, fmt, and clippy pass.
• Review confirms stderr/error propagation and bounded fail-closed readback remain unchanged.

Out of scope

• Snapshot/research logic and Capture full-update Polymarket CLOB evidence for token microstructure research #235 raw CLOB temporal completeness.
• Gate duration, parity policy, health policy, manifest/SHA/_SUCCESS semantics.
• Production cutover; this issue only repairs the candidate behavior needed before a new formal Gate.

Rollout / rollback

Merge independently, build a new exact-main candidate, then run a new 3600+601 second Shadow Gate. Rollback is reverting this single Rust change; production remains on Python until Gate success.