Pass 1: JSON Schema validation against the canonical WPL v1 schema.

Loads and resolves the schema at compile time. Uses ex_json_schema v0.11, which supports Draft 4/6/7. The schema is Draft 2020-12, so we replace the $schema declaration with the Draft 7 URI at load time — the structural keywords used in the WPL schema ($defs, oneOf, if/then/else, additionalProperties, enum, required) are all supported under Draft 7.