Upgrade class fidelity to high-fidelity MATLAB parity

[iahncajigas]

Summary

• upgrade the MATLAB-facing Trial, Fit, Analysis, Decoding, SignalObj, Covariate, and nspikeTrain surfaces into canonical high-fidelity implementations
• add fixture-backed fidelity tests plus broader notebook parity validation and machine-readable notebook parity notes
• refresh the class fidelity audit and parity report so the repo reports semantic fidelity instead of only name mapping

Verification

• pytest -q
• python -m sphinx -W -b html docs docs/_build/html
• python tools/notebooks/run_notebooks.py --group parity_core --timeout 900