Profile per-column cardinality and extent; add data-aware chart warnings (M4, A3/A4)

src/app/components/ChartBuilderModal.tsx

@@ -358,8 +358,14 @@ export function ChartBuilderModal() {
   // fresh array each render (which loops useSyncExternalStore — see SnippetStore note).
   const config = useChartBuilderStore((s) => s.config);
   const rowCount = useChartBuilderStore((s) => s.rowCount);
+  // `columns` is a stable reference set once at init (not rebuilt per render), so
+  // subscribing to it won't loop useSyncExternalStore.
+  const columns = useChartBuilderStore((s) => s.columns);
   const valid = useMemo(() => isBuilderConfigValid(config), [config]);
-  const warnings = useMemo(() => builderWarnings(config, rowCount), [config, rowCount]);
+  const warnings = useMemo(
+    () => builderWarnings(config, rowCount, columns),
+    [config, rowCount, columns],
+  );
   const canSort = useMemo(() => supportsSort(config), [config]);
   const canStack = useMemo(() => supportsStack(config), [config]);