Function map_to_subheading 

pub fn map_to_subheading(
    groups: &[FunctionalGroup],
    organic_class: &OrganicInorganic,
    feat: &StructuralFeatures,
) -> HeadingHint

Derive a 6-digit HS subheading when structural features permit.

Combines functional-group detection with atom counts and ring/bond information. Covers the three most common Chapter 29 groups in chemical trade: ketones (29.14), alcohols (29.05 / 22.07), and carboxylic acids (29.15 / 29.16).

When no 6-digit code can be determined, falls back to map_to_heading (returns subheading: None).