Drop values that cannot be represented as `valueType` instead of refusing to define the field. The client shows the offending rows first.

Date/time fields only, and the only default they can carry: the value is the moment the record was created. A fixed date is almost never the right default, and there is no way to express "now" as a literal.

Written into a record's `customMetadata` when that record is created, so the field is on every new record of its type. This is the only way a definition puts itself on a record: a field with no value is not listed, because a row the record does not carry is a row that lies about it. Never applied to records that already exist.