Tighten nonlinear decoding parity

[iahncajigas]

Summary

• promote StimulusDecode2D to the nonlinear CIF decoding path instead of the regression fallback
• add MATLAB-backed nonlinear CIF and PPDecodeFilter fixtures plus packaging for sympy
• tighten parity audits so notebook and class fidelity reports match runtime behavior

Testing

• pytest -q tests/test_class_fidelity_audit.py tests/test_matlab_symbol_surface.py tests/test_decoding_algorithms_fidelity.py tests/test_matlab_gold_fixtures.py tests/test_notebook_fidelity_audit.py tests/test_notebook_parity_notes.py tests/test_parity_manifest.py tests/test_parity_report.py
• python tools/notebooks/run_notebooks.py --group helpfile_full --topics StimulusDecode2D --timeout 1800
• matlab -batch "cd('/Users/iahncajigas/Library/CloudStorage/Dropbox/Codex/nSTAT'); addpath(fullfile(pwd,'tools','python')); results = runtests('tests/python_port_fidelity'); assertSuccess(results); exit"