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/adapter-dom.sx

@@ -186,10 +186,15 @@
                       (attr-expr (nth args (inc (get state "i")))))
                   (cond
                     ;; Event handler: evaluate eagerly, bind listener
+                    ;; If handler is a 0-arity lambda, wrap to ignore the event arg
                     (starts-with? attr-name "on-")
                       (let ((attr-val (trampoline (eval-expr attr-expr env))))
                         (when (callable? attr-val)
-                          (dom-listen el (slice attr-name 3) attr-val)))
+                          (dom-listen el (slice attr-name 3)
+                            (if (and (lambda? attr-val)
+                                     (= (len (lambda-params attr-val)) 0))
+                              (fn (e) (call-lambda attr-val (list) (lambda-closure attr-val)))
+                              attr-val))))
                     ;; Two-way input binding: :bind signal
                     (= attr-name "bind")
                       (let ((attr-val (trampoline (eval-expr attr-expr env))))