Expose DataFrame operations in the CLI (join, groupby, dedup, sample, transpose, pivot, split)

[bytebrujo]

Problem

The library has powerful join, groupBy, and aggregate ops, but the CLI only exposes count/head/tail/select/filter/sort/stats/convert/validate.

Proposal

Add CLI commands backed by the existing DataFrame engine:

• turbocsv join a.csv b.csv --on key --type inner|left|right|full|cross
• turbocsv groupby col --agg 'salary:mean,id:count'
• turbocsv dedup / unique (optionally by columns)
• turbocsv sample -n 100 / --fraction 0.1
• turbocsv transpose
• turbocsv pivot --index col --columns col --values col
• turbocsv split (by row count or by column value)

Acceptance

• Each command wired into src/cli/index.ts dispatch + help text
• Shell completions updated
• Integration tests per command