Profiler: let users choose which two columns to cross for intersectional gaps

[yakew7]

Summary

Intersectional analysis currently uses only the first two detected dimensions (faircode/SPEC.md §4). Let users pick any two demographic columns to cross.

Why

The most revealing intersection (e.g. race×age) is often not the first two columns in file order.

Tasks

• Web: two dropdowns to select the crossed dimensions, re-render on change
• CLI: --cross colA,colB flag
• Update SPEC.md and keep both engines aligned