Answer architecture round 1: refuse a name read inside the field it binds, check every scope's cycles, build a column read in one pass, drop the implicit struct pick for names across tags, and state the fresh-draw rule once

datatype.go

@@ -77,7 +77,7 @@ func checkColumns(s nodeScope) error {
 		items, _ := columnItems(column)
 		for _, it := range items {
 			if r := it.link.readsColumn; r != nil {
-				if err := check(r.a.head[1:], r.a.tail[0], r.column); err != nil {
+				if err := check(r.category, r.field, r.column); err != nil {
 					return err
 				}
 			}