[Story:] Allow additional top-level attributes in AppDef and RegDef schemas

Context

The AppDef and RegDef JSON schemas set additionalProperties: false on the root object. AppDef
objects legitimately carry extra top-level fields that the schema does not model, for example
baseline and config. The strict root closes the field set and makes schema validation hard-fail
generation on otherwise valid objects. EnvGene does not fully own or know the complete AppDef and
RegDef model, so asserting a closed top-level field set is incorrect.

The schema change is already implemented on branch fix/appdef-regdef-allow-additional-attrs (off
origin/main): the root object is relaxed to additionalProperties: true in schemas/appdef.schema.json,
schemas/regdef.schema.json, schemas/regdef-v2.schema.json, and the two in-package copies under
python/envgene/envgenehelper/schemas/. required fields and the declared types of known fields are
untouched, and nested config blocks stay strict.

In scope changes

1. Review the schema diff.
2. Confirm the test suite is green with the change and add coverage if a gap is found.
3. Release the change.

Review branch fix/appdef-regdef-allow-additional-attrs against origin/main, focusing on schemas/appdef.schema.json, schemas/regdef.schema.json, schemas/regdef-v2.schema.json, and the two copies under python/envgene/envgenehelper/schemas/. Run the test suite, add coverage only if a gap is found, and confirm the reviewed change is ready for release.