test(export): stale-group + same-date ordinal coverage; comment/guard polish

frontend/src/components/ExportDataModal.jsx

@@ -44,7 +44,7 @@ export default function ExportDataModal({
     return () => { alive = false; };
   }, [experimentId]);
 
-  const params = useMemo(() => listExportableParams(dailyTemplate, statuses), [dailyTemplate, statuses]);
+  const params = useMemo(() => listExportableParams(dailyTemplate ?? [], statuses), [dailyTemplate, statuses]);
   const effDataId = dataId ?? params[0]?.id ?? null;
   const dataParam = params.find((p) => p.id === effDataId) ?? params[0] ?? null;
 

frontend/src/lib/dataExport.js

@@ -207,7 +207,7 @@ function escapeCsvCell(value) {
 }
 
 // Serialize a matrix to an RFC-4180-ish CSV string (LF line endings):
-//   <pinned label>,<pinned value>
+//   Data,<metric>
 //   (blank line)
 //   [Group,<group per subject column>]    -- only when groupAxis === 'col'
 //   <corner>[,Group?],<column headers...>
@@ -222,6 +222,8 @@ export function toCSV(matrix) {
     lines.push(groupRow.map(escapeCsvCell).join(','));
   }
 
+  // toCSV is a general serializer; the subject-series builder never emits groupAxis
+  // === 'row' (only 'col' or null), but the branch is retained for that generality.
   const header = matrix.groupAxis === 'row'
     ? ['Group', matrix.corner, ...matrix.columns.map((c) => c.label)]
     : [matrix.corner, ...matrix.columns.map((c) => c.label)];