940737daa7
Decision D from reports/staffing/synthetic-data-gap-report.md §7. Phones in workers_500k.parquet are 11-digit US numbers stored as int64 (e.g. 13122277740). Numerically fine, but breaks join keys against any other source that carries phone as string. Script casts the column to string in place, with non-destructive backup at data/datasets/workers_500k.parquet.bak-<date> before write. Idempotent: if phone is already string, exits 0 with "no-op". Safe to re-run. The .parquet itself is too large to commit (75MB) and follows project convention of staying out of git. The script makes the conversion reproducible from the source dataset.