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]);
 

src/app/infrastructure/dataset-migrations.ts

@@ -10,6 +10,7 @@
 
 import { CURRENT_DATASET_VERSION, type DataSource, type Dataset } from '@core/dataset';
 import type { DataFormat } from '@core/format-detection';
+import type { ColumnStats } from '@core/profile';
 import type { ColumnType } from '@core/type-inference';
 
 const FORMATS: ReadonlyArray<DataFormat> = ['json', 'csv', 'tsv', 'topojson'];
@@ -44,6 +45,10 @@ export function migrateDataset(raw: unknown): Dataset {
     columnTypes: Array.isArray(r.columnTypes)
       ? (r.columnTypes as Array<{ name: string; type: ColumnType }>)
       : [],
+    // Records predating cardinality/extent profiling default to empty stats; the
+    // Chart Builder's data-aware hints simply stay quiet until the dataset is
+    // re-saved (which re-profiles it) — graceful, no forced re-profile on read.
+    columnStats: Array.isArray(r.columnStats) ? (r.columnStats as ColumnStats[]) : [],
     size: typeof r.size === 'number' ? r.size : 0,
     created: typeof r.created === 'string' ? r.created : new Date(0).toISOString(),
     modified: typeof r.modified === 'string' ? r.modified : new Date(0).toISOString(),