fix(data-audit): implement --format both, count validation, and seeded RNG

[BWM0223]

Summary

Fixes #38 - data_audit.ts broken --format both and input validation

Changes

• --format both now correctly writes both JSON and CSV files
• --json / --csv flags removed (consolidated into --format)
• Count validation: non-negative integers only, clear argparse-style error
• SeededRng class for deterministic output with same seed
• Clean separation of validateOptions() and runAudit()

Acceptance Criteria

• --format both creates both .json and .csv files
• Negative counts fail with clear error
• Same seed produces deterministic data