Island guard in aser expansion + page helper IO tests (13 tests)

Fix: islands (defisland) pass component? check but must NEVER be
expanded server-side — they use browser-only reactive primitives
(signal, deref, computed). Added (not (island? comp)) guard in
adapter-sx.sx aser component dispatch.

New test file: shared/sx/tests/test_ocaml_helpers.py
- TestHelperInjection: 5 tests — helper IO proxy, 2-arg calls,
  aser/aser_slot with helpers, undefined helper error
- TestHelperIOPerformance: 2 tests — 20 sequential IO round-trips
  complete in <5s, aser_slot with 5 helpers in <3s
- TestAserSlotClientAffinity: 6 tests — island exclusion, client
  affinity exclusion, server affinity expansion, auto affinity
  behavior in aser vs aser_slot

eval_sx_url stays on bridge.aser() (server-affinity only) for now.
Switching to aser_slot requires fixing the double-aser issue in
_render_to_sx where content gets re-parsed and re-asered.

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

web/adapter-sx.sx

@@ -87,6 +87,7 @@
                   (and comp (macro? comp))
                     (aser (expand-macro comp args env) env)
                   (and comp (component? comp)
+                       (not (island? comp))
                        (or expand-all
                            (= (component-affinity comp) "server"))
                        ;; :affinity :client components are never expanded