Gather the pick key's load and render sides in pick.go, and name each thing for what it is

node.go

@@ -65,9 +65,11 @@ type template struct {
 	nameScope *nameScope // where its tokens look a name up
 
 	// Filled by `resolveTemplates`, from the assembled tree:
-	refs        templateRefs
-	compiled    formatOps
-	readsColumn *columnRead // set when the format only reads one reference or name, and that read is a record's column
+	refs     templateRefs
+	compiled formatOps
+	// readsColumn is set when the format only reads one reference or name, and that read is a
+	// record's column. renderLeaf, proveColumnItem and itemDatatype each read it: change them together.
+	readsColumn *columnRead
 
 	// Filled by `settleRecords`, once the load refused every cycle:
 	columns []recordColumn // a record's, where it has fields, in name order