[Week 1–3] neurosim.loader: real HCP data integration + FMRIPREP confound regression

[TheRoy666]

loader.py is architecturally complete and tested (24 tests passing).
This issue tracks the real-data integration work on June 9 at NBRC.

Tasks:

• Load one HCP S1200 subject via BIDSLoader, confirm X.shape = (400, 1200)
• Verify FMRIPREP confound regression pipeline on real NIfTI
• Confirm DTI connectome loading via load_connectome() on HCP matrices
• Swap Section 1 of notebook 02 from synthetic to real HCP data
• Document any atlas/parcellation mismatches between HCP pipeline and Schaefer-400

Tracks GSoC Weeks 1–3 deliverable.