sx-http: SX request handler — move routing logic from OCaml to SX

New web/request-handler.sx: configurable SX function (sx-handle-request)
that receives path + headers + env and returns rendered HTML.
The handler decides full page vs AJAX fragment.

OCaml server: http_render_page now just calls the SX handler.
All routing, layout selection, AJAX detection moved to SX.
Header parsing added. is_sx_request removed from OCaml.

Configurable via SX_REQUEST_HANDLER env var (default: sx-handle-request).

WIP: handler has parse errors on some URL formats. Needs debugging.

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

tests/playwright/navigation.spec.js

@@ -89,6 +89,30 @@ test.describe('Client-side Navigation', () => {
     expect(headerTextAfter).toContain('sx');
   });
 
+  test('navigation does not create side-by-side layout', async ({ page }) => {
+    await page.goto(BASE_URL + '/sx/(geography)', { waitUntil: 'networkidle' });
+    await page.waitForTimeout(2000);
+
+    // Navigate to Hypermedia
+    await page.click('a[href*="geography.(hypermedia"]:not([href*="example"])');
+    await page.waitForTimeout(3000);
+
+    // The header/nav should NOT be beside the content (side by side)
+    // Check that there's no element with the logo text at x < 300
+    // while content heading is at x > 300
+    const logo = await page.locator('[data-sx-island="layouts/header"]').boundingBox();
+    const heading = await page.locator('h1, h2').first().boundingBox();
+
+    if (logo && heading) {
+      // Both should be roughly centered, not one left and one right
+      const logoCenter = logo.x + logo.width / 2;
+      const headingCenter = heading.x + heading.width / 2;
+      const drift = Math.abs(logoCenter - headingCenter);
+      // If drift > 300px, they're side by side (broken layout)
+      expect(drift).toBeLessThan(300);
+    }
+  });
+
   test('browser back button restores previous page content', async ({ page }) => {
     // Collect console errors
     const errors = [];