Add benchmark Hamiltonian database

[ohno]

Summary

• add a database that maps keys to Hamiltonians and reference energies
• add the internal put! and db() functions
• register three benchmark problems: hydrogen, positronium, and harmonic-oscillator
• add lookup, key discovery, defensive copying, and focused tests
• document database usage, registration, available data, and internal APIs

This pull request was developed with assistance from Codex using GPT-5.6 Sol with High reasoning effort.