host+kernel: blog SPA boost works end-to-end on the WASM OCaml kernel (Playwright 4/4)

Clicking a blog link now fragment-swaps #content with URL push + working back
button, no full reload — the SX-htmx engine driving the same OCaml kernel the
server runs. Six bugs in the source-load + boost path, found by bisecting in
chromium, all fixed:

1. Import double-apply (sx_server.ml x2, sx_browser.ml): the import suspension
   handlers computed `key = library_name_key lib_spec` then called
   `library_loaded_p key` — but library_loaded_p applies library_name_key
   itself, so it ran sx_to_list on a string and crashed ("Expected list, got
   string"). Only unloaded libs suspend, so it only bit lazy imports. Pass the
   spec, not the key.

2. Unloaded-import crash (spec/evaluator.sx + sx_ref.ml library_exports): an
   import of a not-yet-loaded library returned nil exports, and bind-import-set
   did (keys nil) -> crash. Return an empty dict so the import is a graceful
   no-op (lazy symbol resolution covers real usage).

3. value_to_js missing Integer (sx_browser.ml): integers passed to host methods
   were mishandled, so dom-query-all's (host-call node-list "item" i) ignored i
   and returned node 0 for every index — every element aliased the first, so
   only one link ever boosted. Add the Integer -> JS number case.

4. browser-same-origin? rejected relative URLs (browser.sx x2): it only did
   (starts-with? url origin), so "/alpha/" was treated as cross-origin and
   should-boost-link? refused every relative link. Accept scheme-less,
   non-protocol-relative URLs.

5. dom-query-in undefined (orchestration.sx x2): the swap path called a function
   that exists nowhere; it's just dom-query with a container arg.

6. Lazy-deps never loaded under source fallback (sx-platform.js): lazy symbol
   resolution only fires on the VM GLOBAL_GET path, but source-loaded swap
   callbacks run on the CEK and raise instead of lazy-loading, so the post-swap
   hs-boot-subtree!/htmx-boot-subtree! were undefined and aborted URL push.
   Preload the manifest's lazy-deps.

Verified: native host conformance 271/271; lib/host/playwright/spa-check 4/4
(boot, boost, fragment swap + URL push, back button) in real chromium against an
ephemeral durable host server.

lib/host/playwright/spa-check.spec.js

@@ -33,8 +33,10 @@ test.describe('blog SPA', () => {
   test('links inside #content get boosted', async ({ page }) => {
     await page.goto('/');
     await waitReady(page);
-    // the engine marks a boosted element with data-sx-bound containing "boost"
-    await expect(page.locator(POSTLINK).first()).toHaveAttribute('data-sx-bound', /boost/, { timeout: 15000 });
+    // the engine marks a boosted link with the _sxBoundboost JS property
+    await expect
+      .poll(() => page.locator(POSTLINK).first().evaluate((a) => !!a._sxBoundboost), { timeout: 15000 })
+      .toBe(true);
   });
 
   test('clicking a link does a fragment swap — no full reload, URL updates', async ({ page }) => {