Loan-level custom fields, keyed by the custom field's name. Values are typed per each field's definition (string, number, boolean, date, or array for multi-select). Discover available fields via GET /api/custom-fields?context=Loan. On POST/PATCH this object is additive and optional; on PATCH only the supplied keys are updated. Responses list every configured Loan custom field; a field with no value is null.