Extracts categorical values from variable_details recStart/recEnd columns. Handles range notation, special codes, and missing code patterns.
Details
Handles recodeflow notation:
Simple categories: "1", "2", "3"
Integer ranges:
"[7,9]"→ c("7", "8", "9")Continuous ranges: "[18.5,25)" (kept as single value)
Special codes: "copy", "else" (EXCLUDED from mock data generation)
Missing codes: Identified by "NA" in recEnd
Note: "else" is excluded because it acts as a garbage collector in harmonization (recodes to NA::b), not a population category for generation.