Migration and deprecations

Legacy v0 forms are still read but should not be written by current tools: the flat language-named fields julia= / python= / callable= (which historically held inline code, now forbidden) are tolerated on read and rewritten into [<ds>._LANG.<lang>] by an opt-in migrate command; julia_modules / python_includes are retired. Note that bare fetcher/loader, bare shell, and [_LOADERS] are supported forms, not deprecated.

Normative: SCHEMA.md §Deprecations.