Support multi-step derivation chains

[PawanRamaMali]

Summary

Current traceability handles direct ADaM → SDTM mappings. Real submissions often have multi-step chains: SDTM → intermediate ADaM → final ADaM (e.g., ADSL → ADAE derivations).

Tasks

• Extend the tracing algorithm to follow chains of depth > 1
• Visualize derivation graphs (consider DiagrammeR or igraph integration)
• Handle circular references gracefully (detect and warn)
• Add trace_depth parameter to control maximum chain length
• Add test cases for 2-step and 3-step derivation chains

Why

Complex submissions (oncology, rare diseases) have deeply nested derivation chains. Shallow tracing underestimates actual traceability coverage.