Profiler: support an optional reference population baseline (`--reference`)

[yakew7]

Summary

faircode/SPEC.md §2 mentions an optional reference distribution, but it is not implemented. Add a --reference baseline.csv flag (CLI) and an optional reference upload (web) so a dataset can be scored against a real population baseline (e.g. US Census age×sex), not just internal balance.

Why

"Balanced internally" ≠ "representative of the target population." A reference baseline catches under-sampling relative to who the model will actually serve.

Tasks

• Define reference format in SPEC.md (column, expected share)
• Implement in faircode/profiler.py and mirror in assets/profiler-engine.js
• Surface deviation as flags + a per-group expected-vs-actual delta
• Tests in tests/test_profiler.py; keep CLI/web bit-for-bit identical