fix: generate the missing seed art images (broken thumbnails)

The art manifest referenced /art/*.png files that never existed; the SPA
fallback returned index.html for them, so the library thumbnails rendered
as broken images. Generate 5 atmospheric B&W placeholder scenes
(tools/gen-art-placeholders.mjs, npm run gen:art) into public/art/.
tests/tools/art-assets.test.js ties every manifest entry to a real PNG.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/tools/art-assets.test.js

@@ -0,0 +1,18 @@
+import { describe, it, expect } from 'vitest';
+import { readFileSync } from 'node:fs';
+import manifest from '../../src/art/manifest.js';
+
+// Guards the bug we hit: the seed art manifest referenced /art/*.png files that
+// did not exist, so the SPA fallback served index.html and thumbnails broke.
+describe('seed art library assets', () => {
+  it('every manifest image exists as a real PNG (not a broken reference)', () => {
+    expect(manifest.length).toBeGreaterThan(0);
+    for (const asset of manifest) {
+      expect(asset.src.startsWith('/art/'), asset.id).toBe(true);
+      const b = readFileSync('public' + asset.src);          // /art/x.png → public/art/x.png
+      const isPng = b[0] === 0x89 && b[1] === 0x50 && b[2] === 0x4e && b[3] === 0x47;
+      expect(isPng, asset.src + ' is a PNG').toBe(true);
+      expect(b.length, asset.src + ' non-empty').toBeGreaterThan(500);
+    }
+  });
+});