Clarify model structure and add repository contracts

[Thoriri]

Summary

• add shared batch and environment contracts
• add a registry for all five model families
• validate every Hydra experiment in CI
• repair PanLUNA and LuMamba configuration issues
• reorganize onboarding, model, training, and citation documentation
• separate ARES conceptually from the training stack

Testing

• 22 contract and Hydra composition tests
• Python compile check
• documentation link validation
• Apache header validation