Landing: add marketing page at /, move the app to /app/

Multi-page Vite build: index.html serves a new standalone landing
(src/landing), app/index.html serves the app at /app/. The PWA service
worker and manifest are scoped to /app/, so the landing stays
uncontrolled and always-fresh; hash view-state routing is unchanged.

The landing reuses src/core and the chart-renderer service only (never
stores/orchestration/modals/components) and lazy-loads Vega. Its demos
drive the real core: a hero snippet switcher over CHART_EXAMPLES, an
interactive Chart Builder over a new profiled core fixture
(sample-dataset), and a Theme Builder gallery with three from-scratch
custom themes plus built-in presets.

src/core/sample-dataset.test.ts

@@ -0,0 +1,49 @@
+import { describe, it, expect } from 'vitest';
+import { inferColumnType } from '@core/type-inference';
+import {
+  CHART_INTENTS,
+  defaultBuilderConfig,
+  intentApplicable,
+  isBuilderConfigValid,
+} from '@core/chart-builder';
+import {
+  SAMPLE_DATASET_COLUMNS,
+  SAMPLE_DATASET_NAME,
+  SAMPLE_DATASET_ROWS,
+} from '@core/sample-dataset';
+
+describe('sample-dataset fixture', () => {
+  it('declares column types that match what inference derives from the rows', () => {
+    for (const { name, type } of SAMPLE_DATASET_COLUMNS.columnTypes) {
+      const values = SAMPLE_DATASET_ROWS.map((row) => row[name]);
+      expect(inferColumnType(values), name).toBe(type);
+    }
+  });
+
+  it('declares column stats that match the rows', () => {
+    for (const stat of SAMPLE_DATASET_COLUMNS.columnStats ?? []) {
+      const values = SAMPLE_DATASET_ROWS.map((row) => row[stat.name]);
+      expect(stat.distinct, stat.name).toBe(new Set(values).size);
+      const numbers = values.filter((v): v is number => typeof v === 'number');
+      if (numbers.length === values.length) {
+        expect(stat.numericExtent, stat.name).toEqual({
+          min: Math.min(...numbers),
+          max: Math.max(...numbers),
+        });
+      } else {
+        expect(stat.numericExtent, stat.name).toBeNull();
+      }
+    }
+  });
+
+  it('opens the builder on a valid default chart', () => {
+    const config = defaultBuilderConfig(SAMPLE_DATASET_NAME, SAMPLE_DATASET_COLUMNS);
+    expect(isBuilderConfigValid(config)).toBe(true);
+  });
+
+  it('makes every intent applicable, so the demo strip is fully live', () => {
+    for (const intent of CHART_INTENTS) {
+      expect(intentApplicable(intent, SAMPLE_DATASET_COLUMNS), intent).toBe(true);
+    }
+  });
+});