Package-neutral pydantic schema for tokamak data locator catalogs.
The schema describes where a tokamak's data lives across backends (MDSplus
trees, PTData indexes, SQL databases). Other packages (e.g., fdp,
toksearch_d3d) consume the schema; tokamak packages contribute a YAML via
the fdp_schema.catalogs entry-point group.
See docs/ and the design spec in
toksearch_d3d/docs/superpowers/specs/2026-06-01-fdp-schema-design.md.