- ConfigurationModelABC: defaults, src parsing, PascalCase/snake_case
key variants, type casting, required fields, readonly enforcement,
env var override with prefix
- Configuration: set/get by class and string key, auto-instantiation
of unregistered models, add_json_file loading, optional/missing files
Also documents two additional bugs found:
- cast(True, bool) fails with AttributeError (bool has no .lower())
- add_json_file does not exit on invalid JSON (swallows parse error)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>