0052: Selected JSON Schema export

Selected effective Property values, Action input/output, Event output and Context descriptions can be exported as JSON Schema Draft 2020-12 through Catalogue::export_schema and esdieff schema. Effective composition, selectors and alternatives remain library-owned. The separate esdieff.schema/1 report uses json-values fidelity for the documented ordinary JSON subset and refused when that subset cannot represent the selected meaning. Refusal withholds the whole schema, including when an unsupported branch is optional. This avoids giving downstream validators apparently complete schemas with missing meaning.

Choices use anyOf over effective alternatives, since branches can overlap and can replace shared qualities. Nullable widens type admission without weakening const or enum. Numbers retain their exact tokens. Units and defaults are annotations, with no conversions or inserted values. Interaction characteristics and declaration requiredness remain outside selected-value export.

Formats, abstract types, content formats, patterns, extension qualities and Direct items are refused. The existing interpretations do not establish a general equivalent JSON Schema interpretation, particularly for denotation, equality and Direct items nullability. Context still requires explicit nullability. The export contract records supported mappings, limits and examples, grounded in RFC 9880, JSON Schema validation and JSON Schema core.

Direct Rust type generation would require naming and generated-code compatibility policy while still needing separate constraint validation. Lossy schema export would risk silent false admission. A bounded schema bridge lets existing tools serve those uses without either additional contract. A separate exact-arithmetic validator is a development dependency only, for comparison fixtures.