createCustomRecordType

Build the RecordType for one custom record type.

Every custom record shares the custom typeName — one entry in the store's type map covers all of them — but each gets its own validator, keyed to its own type and its own props. That is why this returns a record type per declaration rather than one for the lot: the store validates a record with the type registered under its typeName, and a single shared validator could not tell a review from a checklist.