Add diagnostics for intermittent vibrational regression failures

[harryswift01]

Summary

This PR adds temporary diagnostic output to help identify the source of intermittent CI-only vibrational regression failures.

The goal is to capture the covariance matrices and eigenvalues used in failing vibrational entropy calculations so that local passing runs, CI failing runs, and CI reruns can be compared directly.

Changes

Add vibrational matrix diagnostics:

• Dump united-atom force and torque covariance matrices during regression debug runs.
• Use CODEENTROPY_DEBUG_REGRESSION=1 to enable diagnostic output.
• Write debug files to CODEENTROPY_DEBUG_DIR.

Add eigenvalue diagnostics:

• Dump vibrational entropy input matrices.
• Dump raw eigenvalues before filtering and entropy calculation.
• Label dumped files by matrix type.

Upload regression debug artifacts:

• Enable regression debug mode in the PR regression job.
• Upload regression-debug/** when regression tests fail.

Impact

• Makes CI-only regression failures inspectable instead of relying only on final entropy mismatches.
• Helps determine whether divergence occurs during covariance construction, eigenvalue calculation, or entropy filtering.
• Does not intentionally change production entropy calculations.