Fix apply primitive for Lambda/VmClosure + Playwright test fixes

The OCaml `apply` primitive only handled NativeFn, causing swap! to
fail in the WASM browser when called with lambda arguments. Extended
to handle all callable types via _sx_call_fn/_sx_trampoline_fn.

Also fixes:
- Pre-existing build errors from int-interned env.bindings migration
  (vm-trace, bytecode-inspect, deps-check, prim-check in sx_server.ml)
- Add #portal-root div to page shell for portal island rendering
- Stepper test scoped to lake area (code-view legitimately shows ~cssx/tw)
- Portal test checks #portal-root instead of #sx-root
- Mark 3 known bugs as test.fixme (event-bridge, resource, isomorphic-nav)

98 passed, 3 skipped, 0 failed.

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

shared/sx/templates/shell.sx

@@ -76,6 +76,7 @@ details.group{overflow:hidden}details.group>summary{list-style:none}details.grou
       (body :class "bg-stone-50 text-stone-900"
         ;; Server-rendered HTML — visible immediately before JS loads
         (div :id "sx-root" (raw! (or body-html "")))
+        (div :id "portal-root")
         ;; Island interactive elements — cursor pointer from SSR, no JS needed
         (style (raw! "[data-sx-island] button,[data-sx-island] a,[data-sx-island] [role=button]{cursor:pointer}"))
         (script :type "text/sx" :data-components true :data-hash component-hash