Live conversions need a real mapping

A real CPX user should first load surveys through the Offerwall Ad CPX flow so the stable cpxu_* mapping exists.

Keep the same visitor identity

Do not fetch surveys with one UID and test/complete them using an unrelated external user ID.

Dashboard test behavior

The CPX dashboard can send a dummy user for URL validation. The receiver can acknowledge an unmapped test callback without crediting money; this does not make arbitrary live users creditable.