extras-map-one-to-one-to-modules

Summary

Each optional dependency group in the package corresponds to exactly one module file, so installing the "pg" extra unlocks the Postgres module, "cluster" unlocks the clustering module, and "test-pg" bundles both Postgres and test dependencies together. This means you only pull in the code you can actually use, and the mapping is predictable — if you know which module you need, you know which extra to install.