Add C-PAC vs RBC comparison script

Compares pipeline outputs step-by-step (Dice for masks, spatial
correlation for volumes, per-column correlation for regressors,
element-wise correlation for connectivity matrices). Generates a
markdown report with figures and a terminal summary.

Supports both glob-based file discovery and manifest mode
(--manifest tests/full_pipeline/.last_run.json) for comparing
against raw pipeline test outputs.

Author: nx10