Fix ListRef handling in streaming data — list from SX is ListRef in OCaml

The streaming render matched `List items` but SX's `(list ...)` produces
`ListRef` (mutable list) in the OCaml runtime. Data items were rejected
with "returned list, expected dict or list" — 0 resolve chunks sent.

Fixed both streaming render and AJAX paths to handle ListRef.
Added sandbox test for streaming-demo-data return type validation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/playwright/streaming.spec.js

@@ -263,6 +263,34 @@ test.describe('Streaming sandbox', () => {
     await expect(page.locator('[data-suspense="stream-slow"]')).toContainText('Slow resolved!');
   });
 
+  test('streaming-demo-data returns iterable list of dicts with stream-ids', async ({ page }) => {
+    const errors = await bootSandbox(page);
+    expect(errors).toEqual([]);
+
+    // Verify the data function returns a list that can be iterated
+    // (catches ListRef vs List type mismatch)
+    const result = await page.evaluate(() => {
+      const K = window.SxKernel;
+      try {
+        const type = K.eval('(type-of (streaming-demo-data))');
+        const len = K.eval('(len (streaming-demo-data))');
+        const ids = K.eval('(join "," (map (fn (item) (get item "stream-id")) (streaming-demo-data)))');
+        const delays = K.eval('(join "," (map (fn (item) (str (get item "delay"))) (streaming-demo-data)))');
+        return { type, len, ids, delays };
+      } catch(e) {
+        return { error: e.message };
+      }
+    });
+
+    expect(result.error || '').toBe('');
+    expect(result.type).toBe('list');
+    expect(result.len).toBe(3);
+    expect(result.ids).toContain('stream-fast');
+    expect(result.ids).toContain('stream-medium');
+    expect(result.ids).toContain('stream-slow');
+    expect(result.delays).toContain('1000');
+  });
+
   test('streaming shell renders with outer layout gutters', async ({ page }) => {
     const errors = await bootSandbox(page);
     expect(errors).toEqual([]);