feat(frontend): extractValue helper for data export

frontend/src/lib/dataExport.js

@@ -0,0 +1,23 @@
+// Pure helpers for exporting daily-parameter data as a date×subject CSV matrix.
+// No React, no I/O — mirrors the crossSubjectChart.js pure-helper pattern.
+
+// Read a single parameter's raw value from one daily status.
+// Returns null when the value is absent. Note: 0 and '' are returned as-is
+// (real values); only genuinely-missing lookups become null.
+export function extractValue(status, param) {
+  if (!status || !param) return null;
+  switch (param.kind) {
+    case 'total':
+      return status.analysis_summary?.total ?? null;
+    case 'success_rate':
+      return status.analysis_summary?.success_rate ?? null;
+    case 'category':
+      return status.analysis_summary?.counts?.[param.category] ?? null;
+    case 'builtin':
+      return status[param.statusKey] ?? null;
+    case 'custom':
+      return status.custom_fields?.[param.fieldId] ?? null;
+    default:
+      return null;
+  }
+}