Term expansion (square / cross / cubic) and per-variable scaling UI

[kgdunn]

Goal

Complete the preprocessing recipe with model-term expansion and explicit per-variable scaling controls.

Scope

• Extend PreprocessingSpec / apply_spec to build expanded terms (square, cross, cubic) from selected X variables.
• UI to create and remove expanded terms.
• Per-variable scaling controls (unit variance / pareto / none) exposed in the explorer, surfacing the scaling map already in the spec.
• Show transformed/scaled previews consistently with the existing raw/scaled toggle.

Roadmap item — keep open.