Creates a ThinkLP investigation case and links a batch of Frontier point-of-sale transactions to it as snapshots.

The API resolves each transaction's employee to a canonical Person and attaches that Person to the case as an involved person. When a Contact has no canonical Person, the API provisions one. When an employee cannot be resolved, the request fails.

The API also provisions each line item as a canonical Item and attaches it as a case item. It aggregates line items case-insensitively by SKU.

The whole operation is transactional. Any failure rolls back the case and every link. A write that the platform rejects returns 400 INVALID_CASE_PAYLOAD, and the message names the platform status code and the rejected fields. A duplicate transaction id returns 409 ALREADY_LINKED instead.