Function classify_smiles 

pub fn classify_smiles(smiles: &str) -> JsValue

Analyse a SMILES string and return functional-group + HS heading hint.

Returns a SmilesClassification JS object, or null if the SMILES string is empty or cannot be parsed.

JS return shape

{
  "organic_class": "organic",
  "functional_groups": ["CarboxylicAcid"],
  "heading_hint": {
    "chapter": 29,
    "heading": 2915,
    "rationale": "Carboxylic acid → heading 29.15",
    "confidence": 0.60
  }
}