Add deterministic regression harness

[yingyue2030699]

Summary

• Add a deterministic regression harness for upgrade validation.
• Provide repeatable checks for comparing expected simulation behavior.

Validation

• Run the regression harness commands documented in the branch.

[yingyue2030699]

Mergeability review:

1. Content: Mostly looks aligned with the stated goal. The PR adds tests/regression/README.md, tests/regression/manifest.json, and tests/regression/run_regression.py, providing a deterministic self-baselining regression harness with fresh-run and restart cases.
2. Verification: Not verified. Running python3 tests/regression/run_regression.py --build on the PR branch fails at make serial because the branch predates the serial build wiring fixes. I then tested the merged result in a temporary worktree from current origin/master plus this PR; that also fails at make serial with a duplicate Parameters::bondedComplexWrite member in include/classes/class_Parameters.hpp. Since the harness cannot build and run on the target branch state, correctness is not verified.
3. Dependency: No unfinished dependent PR is blocking this now; the earlier smoke-runner stack base has merged. The blocker is the current target branch build state, not an unmerged dependency.

Decision: Not mergeable right now. Halting here until master is repaired and the regression harness can be verified on the merged result.