More convert targets: Parquet, Arrow/IPC, Excel (.xlsx)

[bytebrujo]

Problem

src/cli/commands/convert.ts supports JSON/JSONL/TSV/CSV only. Real data tooling interop needs columnar/spreadsheet formats.

Proposal

Add read + write support for:

• Parquet (columnar, most requested)
• Arrow / IPC (zero-copy interop)
• Excel .xlsx

turbocsv convert --to parquet data.csv -o data.parquet
turbocsv convert --to xlsx data.csv -o data.xlsx

Notes

• Evaluate whether to implement in Zig vs. lean on a JS lib for xlsx.
• Likely split into one sub-issue per format.

Acceptance

• At least Parquet write + read round-trips
• CLI flag + tests