Fix: missing error handling of ground truth labels with a slash

[mducducd]

• Replace / with _ at every filename-construction site in the statistics phase: ROC/PR curve SVGs (single- and multi-target) and the categorical/regression stats CSVs. missing error handling of ground truth labels with a slash #176
• Original label is preserved in CSV columns, plot titles, and config — only the filename component is sanitized.
• Refactor log.info in data.py and redundant code in cox.py