feat(builtins): add csv builtin (qsv-inspired CSV utilities)

[chaliy]

Goal

Add native CSV/TSV handling for common data workflows that are awkward with pure awk or jq.

Proposal

Add a focused csv builtin inspired by qsv/xsv, optimized for the operations agents and scripts hit most often.

Initial subcommands

• csv headers <file>
• csv select <cols> <file>
• csv to-json <file>
• csv from-json <file>
• csv sort <col> <file>
• csv join <left-col> <right-col> <left> <right>
• csv stats <file>

Format support

• CSV first
• TSV via delimiter flag
• stdin/stdout friendly
• proper quoting / escaping / embedded commas / newlines

Why this shape

A CSV-specific builtin covers the common structured-tabular workflows without pulling in the full surface area of a tool like Miller. For more complex transforms, users can compose csv to-json | jq ... | csv from-json.

Non-goals

• full spreadsheet language
• trying to clone every mlr feature in v1

Tests

• quoted cells
• escaped quotes
• delimiter override
• headerless mode
• join on missing keys
• malformed CSV errors
• JSON round-trip compatibility

[chaliy]

Resolved in PR #585 or #588