SMILES Sanity Check

Checks a SMILES string token by token against the SMILES grammar — balanced brackets, atom symbols on the organic subset or a bracketed element, legal bond tokens, paired ring-closure digits, bond-order sums within normal valence — and returns a per-check VALID/INVALID verdict so copy-paste corruption and hallucinated structures are caught before a molecule is quoted. Use whenever a SMILES string appears in a paper, patent, dataset or prompt — "is this SMILES valid?", "sanity-check these SMILES strings", "does this SMILES parse?", "validate the smiles column". Not for prose molecular claims such as formulas or molecular weights (use `chemistry-claim-check`), and not a sub…