COCONUT 2.0 candidate data attribution

This structure-only candidate pool is derived from the upstream Kaggle dataset [COCONUT 2.0 candidates + fingerprints (CASMI26)](https://www.kaggle.com/datasets/prvsiyan/coconut-casmi26-candidates), whose metadata declares CC BY 4.0. COCONUT 2.0 is by Chandrasekhar et al. Preserve the attribution and license when redistributing.

The upstream data is a candidate-structure/mass/fingerprint resource, not an MS/MS spectrum library. This compact derivative contains only InChIKey14, SMILES, and exact mass. Provenance and SHA-256 hashes are in coconut-manifest.json.