Profiler CLI: expose thresholds as flags (`--min-share`, `--imbalance-flag`, …)

[yakew7]

Summary

The constants in faircode/SPEC.md §7 (MIN_SHARE_THRESHOLD, INTERSECTION_FLOOR, IMBALANCE_FLAG, MISSING_FLAG) are hard-coded. Expose them as CLI flags.

Why

Different domains have different "under-represented" bars; auditors should be able to tune without editing source.

Tasks

• Add argparse flags with the SPEC defaults
• Thread through profiler.profile(...) as parameters
• Document defaults in --help and README