turbocsv diff: key-aware diff between two CSV files

## Proposal
Row/cell-level diff between two CSVs, useful for data pipelines and PR review:

```bash
turbocsv diff old.csv new.csv --key id
```

Report added / removed / changed rows, and per-cell changes for matched keys.

## Acceptance
- [ ] Key-based matching with added/removed/changed classification
- [ ] Human-readable + machine-readable (json) output
- [ ] Tests