Add traceability coverage report export

[PawanRamaMali]

Summary

The traceability engine computes ADaM-to-SDTM variable chains, but there is no structured export format for inclusion in regulatory submission packages.

Tasks

• Add export_traceability_report() that produces a formatted table suitable for ADRG appendices
• Support output formats: data frame, CSV, and optionally Excel (via writexl)
• Include coverage metrics per ADaM dataset (% variables traced, untraced variables listed)
• Add a summary row with overall traceability coverage percentage

Why

Traceability documentation is a regulatory requirement. A structured export bridges R4SUB analysis to actual submission documents.