Page helpers demo: defisland, map-in-children fix, _eval_slot ref evaluator

- Add page-helpers-demo page with defisland ~demo-client-runner (pure SX,
  zero JS files) showing spec functions running on both server and client
- Fix _aser_component children serialization: flatten list results from map
  instead of serialize(list) which wraps in parens creating ((div ...) ...)
  that re-parses as invalid function call. Fixed in adapter-async.sx spec
  and async_eval_ref.py
- Switch _eval_slot to use async_eval_ref.py when SX_USE_REF=1 (was
  hardcoded to async_eval.py)
- Add Island type support to async_eval_ref.py: import, SSR rendering,
  aser dispatch, thread-first, defisland in _ASER_FORMS
- Add server affinity check: components with :affinity :server expand
  even when _expand_components is False
- Add diagnostic _aser_stack context to EvalError messages
- New spec files: adapter-async.sx, page-helpers.sx, platform_js.py
- Bootstrappers: page-helpers module support, performance.now() timing
- 0-arity lambda event handler fix in adapter-dom.sx

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/run_js_sx.py

@@ -103,8 +103,11 @@ def compile_ref_to_js(
     if "boot" in adapter_set:
         spec_mod_set.add("router")
         spec_mod_set.add("deps")
+        if "page-helpers" in SPEC_MODULES:
+            spec_mod_set.add("page-helpers")
     has_deps = "deps" in spec_mod_set
     has_router = "router" in spec_mod_set
+    has_page_helpers = "page-helpers" in spec_mod_set
 
     # Resolve extensions
     ext_set = set()
@@ -198,12 +201,12 @@ def compile_ref_to_js(
         if name in adapter_set and name in adapter_platform:
             parts.append(adapter_platform[name])
 
-    parts.append(fixups_js(has_html, has_sx, has_dom, has_signals, has_deps))
+    parts.append(fixups_js(has_html, has_sx, has_dom, has_signals, has_deps, has_page_helpers))
     if has_continuations:
         parts.append(CONTINUATIONS_JS)
     if has_dom:
         parts.append(ASYNC_IO_JS)
-    parts.append(public_api_js(has_html, has_sx, has_dom, has_engine, has_orch, has_boot, has_parser, adapter_label, has_deps, has_router, has_signals))
+    parts.append(public_api_js(has_html, has_sx, has_dom, has_engine, has_orch, has_boot, has_parser, adapter_label, has_deps, has_router, has_signals, has_page_helpers))
     parts.append(EPILOGUE)
 
     build_ts = datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ")