Stepper persistence: use def-store instead of broken cookies

The home stepper's step-idx signal was not persisting across SX
navigation because set-cookie/freeze-to-sx wasn't working in the
WASM kernel. Replace with def-store which uses a global registry
that survives island re-hydration.

Also fix sx_http.exe build: add sx_http back to dune, inline scope
primitives (Sx_scope module was removed), add declarative form
stubs and render stubs, fix /sx/ home route mapping.

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

tests/playwright/navigation.spec.js

@@ -125,6 +125,46 @@ test.describe('Page Navigation', () => {
     expect(after).toContain('geography');
   });
 
+  test('stepper persists index across navigation', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(3000);
+
+    // Get the initial stepper index
+    const getIndex = () => page.evaluate(() => {
+      const el = document.querySelector('[data-sx-island="home/stepper"]');
+      const m = el && el.textContent.match(/(\d+)\s*\/\s*\d+/);
+      return m ? parseInt(m[1]) : null;
+    });
+
+    const initial = await getIndex();
+    expect(initial).not.toBeNull();
+
+    // Advance the stepper
+    await page.evaluate(() => {
+      const btns = document.querySelectorAll('[data-sx-island="home/stepper"] button');
+      if (btns.length >= 2) btns[1].click();  // next button
+    });
+    await page.waitForTimeout(500);
+
+    const advanced = await getIndex();
+    expect(advanced).toBe(initial + 1);
+
+    // Navigate away
+    await page.click('a[sx-get*="(geography)"]');
+    await page.waitForTimeout(2000);
+
+    // Navigate back home
+    await page.evaluate(() => {
+      const link = document.querySelector('a[sx-get*="/sx/"]');
+      if (link) link.click();
+    });
+    await page.waitForTimeout(3000);
+
+    // Stepper should still show the advanced index
+    const afterNav = await getIndex();
+    expect(afterNav).toBe(advanced);
+  });
+
   test('header island renders with SSR', async ({ page }) => {
     await page.goto(BASE_URL + '/sx/(geography)', { waitUntil: 'networkidle' });