fix(ci): flatten policy approval download

[rylinjames]

Summary

• flatten the exact approval artifact into the verifier's declared target directory
• add regression coverage for the ordinary protected-ratchet download layout

Live evidence

Protected run 33165642340 completed successfully and the ordinary verifier accepted its status, run, and artifact ID. Attempt 2 of ordinary run 33165637778 then failed only because download-artifact placed quality-policy-approval.json under an artifact-ID subdirectory. This applies the documented merge-multiple flattening already used by the protected validation path.

Validation

• 16 focused quality-ratchet tests pass
• workflow YAML parses
• focused Ruff and diff checks pass

Final bootstrap follow-up for #291. The self-protected ratchet failure is expected; all ordinary checks must pass before audited admin merge.

[rylinjames]

Independent review: APPROVE. The reviewer confirmed merge-multiple only flattens the already-selected single artifact ID; exact artifact name/unexpired/digest, repository, run ID, candidate/base SHA, workflow, and validation-artifact bindings remain unchanged and fail closed. Live attempt 2 selected artifact 9683482457 but extracted under its name with merge=false. Reviewer validation: 16 tests, Ruff format/check, YAML parse, and diff check passed.

[rylinjames]

Audited protected-workflow bootstrap: independent review is APPROVE and all ordinary pytest, doctor, and wheel checks are green. The sole failure is expected self-protection because this PR changes the protected ordinary ratchet workflow. Proceeding with an explicit admin squash merge without changing branch protection.