ci: revert same-runner identity parallelization

[abrichr]

What changed

• restore the required test job to the single-process suite from before ci: parallelize profiled identity regressions #231
• remove the parallel-lane workflow contract that is no longer applicable
• restore the reviewed public-artifacts.json hash for the CI workflow

This is an exact restoration of the three affected files from #230 commit
5857498fa4714dfb09413eebb5ff518a8ff1ac34. It preserves the two-speed
nightly/release matrix routing, all tests, and the unchanged 85% safety-path
coverage floor.

Root cause

Post-merge run
30007710974
failed in required job test. The all-config identity-ladder harness timed out
after 900 seconds inside RapidOCR/ONNX while the normal suite and the heavy
identity lane competed concurrently on the same runner. The normal lane still
completed with 2,966 passing tests, and browser E2E passed.

The parallel layout was green on the PR head but was not stable enough for the
required main gate. Serial execution is the conservative fix-forward.

Validation

• actionlint .github/workflows/ci.yml
• pytest -q tests/test_ci_workflow_contract.py tests/test_release_ci_gate.py tests/test_release_contract.py — 48 passed
• exact three-file content/tree match against 5857498fa4714dfb09413eebb5ff518a8ff1ac34
• fresh wheel and sdist build
• check_release_consistency.py --validate-dist-dir ... --license-file LICENSE
• git diff --check

No product/runtime code, tests, coverage thresholds, release routing, or branch
protection settings are changed.