[thorwhalen]

Closes the systemic fix recommended in #62.

• New lacing/json_schema.py: DefaultFactoryJsonSchema, a GenerateJsonSchema subclass that writes default for every zero-argument default_factory field whose result is JSON-serialisable (factories that take validated data, or return something unencodable, are left alone); and SchemaModel, a BaseModel whose model_json_schema() uses it.
• lacing.schema.json_schema() / export_json_schemas() always use the generator, so any registered body gets it whatever its base class. The in-repo bodies and envelope models (Provenance, Annotation, refs) now inherit SchemaModel, so a plain Model.model_json_schema() agrees with the committed artifacts.
• character-voice/v1 loses its per-field json_schema_extra={"default": {}} (character-voice/v1: provider_voice_ids exports no JSON-Schema default, so zod mirrors parse it as undefined instead of {} #40): the generator emits the same default, and its committed schema is byte-identical.
• Regenerated lacing/schema/*: exactly three additions — review/v1 target_annotation_ids [], continuity-violation/v1 evidence {}, reference-lock/v1 params {}. A schema default never invalidates a document (the fields stay optional), so no major bump; downstream zod mirrors will now parse an absent value as [] / {} instead of undefined.
• Provenance.was_derived_from (envelope) now exports [] too. The envelope is not part of lacing/schema/* (only bodies are exported), so no committed file changes for it.
• tests/test_json_schema.py: for every registered body, each zero-argument default_factory field exports a default equal to the model's, and stays optional; the three reported fields; committed schemas equal a fresh export (all bodies, not only character-voice); the generator on plain BaseModels; the unencodable/data-taking cases.

lacing-ui's codegen freshness gate will show the three-line mirror diff once it regenerates (tracked on reeleehq/lacing-ui#8).

Closes #62

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