Add stochastic regression validation

[yingyue2030699]

Summary

• Add stochastic seed-set checks on top of the deterministic regression harness.
• Capture validation coverage for stochastic upgrade behavior.

Stack

• Base branch: codex/upgrade-regression-harness

Validation

• Run the stochastic validation commands documented in the branch.

[yingyue2030699]

Mergeability review:

1. Content: Looks generally aligned with the stated goal. Relative to codex/upgrade-regression-harness, the PR extends tests/regression/README.md, tests/regression/manifest.json, and tests/regression/run_regression.py with stochastic seed-set validation support.
2. Verification: Not verified. GitHub reports no checks for this branch, and the documented validation cannot be run cleanly because its base dependency, PR Add deterministic regression harness #7 (codex/upgrade-regression-harness), is not merged and is currently blocked by a build failure on the target branch state.
3. Dependency: Blocked. This PR is stacked on codex/upgrade-regression-harness / PR Add deterministic regression harness #7, and PR Add deterministic regression harness #7 is not complete yet.

Decision: Not mergeable right now. Halting here until PR #7 is repaired, verified, and merged.