Fix WASM kernel deploy: 3.6MB js_of_ocaml → 68KB wasm_of_ocaml

The deployed sx_browser.bc.wasm.js was actually the js_of_ocaml output
(pure JS), not the wasm_of_ocaml loader. Nothing synced the correct
build output from _build/ to shared/static/wasm/.

- sx_build target=ocaml now auto-syncs WASM kernel + JS fallback + assets
- sx-build-all.sh syncs after dune build
- Correct 68KB WASM loader replaces 3.6MB JS imposter

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

tests/playwright/navigation.spec.js

@@ -149,6 +149,53 @@ test.describe('Page Navigation', () => {
     expect(afterNav).toBe(advanced);
   });
 
+  test('sx-get link fetches SX not HTML and preserves layout', async ({ page }) => {
+    await loadPage(page, '');
+
+    // Track requests to detect what format the fetch uses
+    const fetches = [];
+    page.on('request', req => {
+      if (req.url().includes('/sx/') && req.resourceType() === 'xhr')
+        fetches.push({ url: req.url(), accept: req.headers()['accept'] || '' });
+    });
+
+    // Mark page so we can detect full reload vs SPA nav
+    await page.evaluate(() => window.__spa_marker = true);
+
+    // Click a nav link (Geography)
+    await page.click('a[sx-get*="(geography)"]');
+    await expect(page).toHaveURL(/geography/, { timeout: 5000 });
+
+    // Must be SPA navigation — marker survives
+    const marker = await page.evaluate(() => window.__spa_marker);
+    expect(marker).toBe(true);
+
+    // The fetch should have been XHR, not a document navigation
+    expect(fetches.length).toBeGreaterThan(0);
+
+    // After navigation, #sx-nav and #main-content should still be
+    // vertically stacked (not side-by-side). Check that nav is above content.
+    const layout = await page.evaluate(() => {
+      const nav = document.querySelector('#sx-nav');
+      const main = document.querySelector('#main-content, #main-panel');
+      if (!nav || !main) return { error: 'missing elements', nav: !!nav, main: !!main };
+      const navRect = nav.getBoundingClientRect();
+      const mainRect = main.getBoundingClientRect();
+      return {
+        navBottom: navRect.bottom,
+        mainTop: mainRect.top,
+        navRight: navRect.right,
+        mainLeft: mainRect.left,
+        // Nav should end before main starts (vertically stacked)
+        verticallyStacked: navRect.bottom <= mainRect.top + 5,
+        // Nav and main should overlap horizontally (not side-by-side)
+        horizontalOverlap: navRect.left < mainRect.right && mainRect.left < navRect.right
+      };
+    });
+    expect(layout.verticallyStacked).toBe(true);
+    expect(layout.horizontalOverlap).toBe(true);
+  });
+
   test('header island renders with SSR', async ({ page }) => {
     await loadPage(page, '(geography)');