Sign up an end customer for a product

Creates an end customer's own login for one company: the credential, the underlying legal entity, the customer, and a pending enrollment in the named Product, then signs them in. The entity starts pending and the enrollment starts pending, exactly as customer.create leaves them. Verification is the customer's own act, never a side effect of signing up. No account is created and no money can move until the entity clears and the enrollment is activated. The email is the login handle and is unique per company, so the same person using two of a company's products is one customer with one password. The session token is a one-time secret surfaced only in this response, never in receipts or replays.