Format plugin architecture for multi-format I/O

[vmvarela]

Part of #68

Description

Refactor the current CSV reader/writer into a pluggable format interface so new formats can be added incrementally. This is the prerequisite for all other #68 sub-issues.

Acceptance Criteria

• Internal reader/writer interface extracted (e.g. FormatReader, FormatWriter types or function pointers)
• CSV remains the default format — no behavior change for existing users
• --input-format <fmt> / -I <fmt> flag accepted (currently: only csv valid)
• --output-format <fmt> / -O <fmt> flag accepted (currently: only csv valid)
• Unknown format values produce a clear error message listing supported formats
• Existing --tsv and --json flags continue to work (mapped to --output-format tsv/json internally)

Notes

• Keep it simple: a tagged union or vtable-style dispatch in Zig is sufficient
• Don't remove --tsv / --json flags yet — they can be aliases mapped to --output-format
• This issue is purely architectural; no new format support is added here