test(engine): guard against React/DOM imports in engine

src/engine/__tests__/purity.test.ts

@@ -0,0 +1,26 @@
+import { readdir, readFile } from 'node:fs/promises';
+import { join } from 'node:path';
+import { describe, expect, it } from 'vitest';
+
+const ENGINE_DIR = join(import.meta.dirname, '..');
+const FORBIDDEN = [/from\s+['"]react/, /from\s+['"]react-dom/, /from\s+['"]zustand/];
+
+async function engineSourceFiles(): Promise<string[]> {
+  const entries = await readdir(ENGINE_DIR, { withFileTypes: true });
+  return entries
+    .filter((e) => e.isFile() && e.name.endsWith('.ts') && !e.name.endsWith('.test.ts'))
+    .map((e) => join(ENGINE_DIR, e.name));
+}
+
+describe('engine purity', () => {
+  it('does not import React, react-dom, or Zustand', async () => {
+    const files = await engineSourceFiles();
+    expect(files.length).toBeGreaterThan(0);
+    for (const file of files) {
+      const source = await readFile(file, 'utf8');
+      for (const pattern of FORBIDDEN) {
+        expect(source, `${file} must stay free of ${pattern}`).not.toMatch(pattern);
+      }
+    }
+  });
+});