fix: skip Attribute fallback for custom-content datagrid columns

PR #202 added a sugar that falls back to the column's name when no
explicit Attribute is given. For custom-content columns (those with
an action-button body and no attribute binding), this caused CE1613
"attribute no longer exists" because names like 'colActions' don't
resolve to real attributes.

Skip the sugar when the column has child widgets — those columns
don't bind to an attribute.

Fixes the 03-page-examples.mdl integration test regression.

Author: ako

mdl/executor/cmd_pages_builder_v3_widgets.go

@@ -128,10 +128,9 @@ func (pb *pageBuilder) buildDataGridV3(w *ast.WidgetV3) (*pages.CustomWidget, er
 			attr := child.GetAttribute()
 			// Sugar: when no explicit Attribute: property is given, fall back to
 			// the column's name. This lets `COLUMN Sku (Caption: 'SKU')` work
-			// without repeating `Attribute: Sku`. If the name starts with a
-			// lowercase prefix like "col" (convention for decoration), it won't
-			// resolve to a real attribute — and mx check will flag it later.
-			if attr == "" && child.Name != "" {
+			// without repeating `Attribute: Sku`. Skip for custom-content columns
+			// (those with a body of child widgets), which don't bind to an attribute.
+			if attr == "" && child.Name != "" && len(child.Children) == 0 {
 				attr = child.Name
 			}
 			col := ast.DataGridColumnDef{