[Feat]: Add support for CO, RELREC, RELSPEC, RELSUB domains

[rubentalstra]

Description

Add support for SDTM IG 3.4 special-purpose domains: CO (Comments), RELREC (Related Records), RELSPEC (Related Specimens), RELSUB (Related Subjects).

Location

• Files: standards/sdtm/ig/v3.4/
• Files: crates/tss-standards/src/sdtm/

Current Behavior

These special-purpose domains are not defined in the standards data.

Expected Behavior

• Domain definitions for CO, RELREC, RELSPEC, RELSUB
• Variable specifications per SDTM IG 3.4
• Validation rules for these domains

Acceptance Criteria

• CO domain definition and variables
• RELREC domain definition and variables
• RELSPEC domain definition and variables
• RELSUB domain definition and variables
• Validation rules implemented
• Tests pass