Split PropertyField/RecordField on parent-kind instead of nullable column

ChildField.PropertyField and ChildField.RecordField each carry both columnName: String and column: ColumnRef, with column nullable depending on the parent type: non-null when the parent is a JooqTableRecordType with a resolvable column, null for JooqRecordType / JavaRecordType / PojoResultType parents. The single record straddles two parent kinds via an Optional component, leaving columnName as the only carrier of the SDL string when column is absent. Per Narrow component types over broad interfaces and Sub-taxonomies for resolution outcomes, the right shape is two sealed-arm variants (one for table-backed parents carrying a non-null ColumnRef, one for non-table-backed parents carrying just the SDL string), not one record with a nullable component. Split surfaced during R50’s columnName cleanup on ChildField.ColumnField / ColumnReferenceField, where the table-backed-only invariant let those carriers retire columnName outright; this item carries the same rigour to PropertyField and RecordField.