Add CSV parsing and generation builtins.

Motivation

Tabular data is one of the most common data formats agents process. CSV parsing with proper quoting, escaping, and header handling is needed for data analysis, report generation, and ETL tasks.

Proposed functions

• csv_parse(@String -> Array<Array<@String>>) — parse CSV string into rows of fields
• csv_parse_with_headers(@String -> Array<Map<@String, @String>>) — parse with header row
• csv_format(Array<Array<@String>> -> @String) — generate CSV from rows

Implementation notes

RFC 4180 compliant. Handles quoted fields, embedded commas, embedded newlines. Pure functions.

Dependencies

• Standard library collections (Set, Map, Decimal) #62 (collections) — csv_parse_with_headers needs Map type
• Array operations (map, fold, slice) #133 (array operations) — useful for processing parsed CSV data