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>

sx/sx/nav-data.sx

@@ -227,7 +227,8 @@
   (dict :label "JavaScript" :href "/bootstrappers/javascript")
   (dict :label "Python" :href "/bootstrappers/python")
   (dict :label "Self-Hosting (py.sx)" :href "/bootstrappers/self-hosting")
-  (dict :label "Self-Hosting JS (js.sx)" :href "/bootstrappers/self-hosting-js")))
+  (dict :label "Self-Hosting JS (js.sx)" :href "/bootstrappers/self-hosting-js")
+  (dict :label "Page Helpers" :href "/bootstrappers/page-helpers")))
 
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