Add lazy recursive schemas with DTO and JSON Schema export

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What this task asks for

Add a lazy schema type for self-referencing recursive data with full serialization, validation, and export support. DynamoDB commonly stores recursive data but users modeling these structures must use `any()`, losing type safety, validation, conditions, updates, and exports. Add a `lazy()` schema enabling self-referencing definitions. `lazy()` accepts a thunk returning a Schema, producing a schema with `type` `'lazy'`, cached single-execution `resolve()`, and the same builder interface as other schema types. Invalid resolution causes `check()` to throw `schema.lazy.invalidResolution`. DTO serialization replaces each recursive reference with a bare object containing only a `$ref` key and no `type` field. Deserialized schemas must parse data identically to the original.