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/sxc/pages/docs.sx

@@ -553,6 +553,28 @@
       "phase2" (~reactive-islands-phase2-content)
       :else (~reactive-islands-index-content))))
 
+;; ---------------------------------------------------------------------------
+;; Bootstrapped page helpers demo
+;; ---------------------------------------------------------------------------
+
+(defpage page-helpers-demo
+  :path "/bootstrappers/page-helpers"
+  :auth :public
+  :layout :sx-docs
+  :data (page-helpers-demo-data)
+  :content (~sx-doc :path "/bootstrappers/page-helpers"
+    (~page-helpers-demo-content
+      :sf-categories sf-categories :sf-total sf-total :sf-ms sf-ms
+      :ref-sample ref-sample :ref-ms ref-ms
+      :attr-result attr-result :attr-ms attr-ms
+      :comp-source comp-source :comp-ms comp-ms
+      :routing-result routing-result :routing-ms routing-ms
+      :server-total-ms server-total-ms
+      :sf-source sf-source
+      :attr-detail attr-detail
+      :req-attrs req-attrs
+      :attr-keys attr-keys)))
+
 ;; ---------------------------------------------------------------------------
 ;; Testing section
 ;; ---------------------------------------------------------------------------