[nodivbyzero]

Summary

• Add the codata subpackage with selected 2022 CODATA constants.
• Add the opt-in parse subpackage for parsing values such as 5 km and 10lb.
• Support built-in unit registries and custom parser registries.
• Add tests for constants, built-in parsing, and custom dimensions.
• Document both optional packages in the README.

Design

String parsing remains outside the core package, preserving the zero-registry core API. CODATA values are exposed as documented float64 constants without uncertainty or correlation modeling.