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/platform_py.py

@@ -1428,6 +1428,7 @@ SPEC_MODULES = {
     "router": ("router.sx", "router (client-side route matching)"),
     "engine": ("engine.sx", "engine (fetch/swap/trigger pure logic)"),
     "signals": ("signals.sx", "signals (reactive signal runtime)"),
+    "page-helpers": ("page-helpers.sx", "page-helpers (pure data transformation helpers)"),
 }
 
 EXTENSION_NAMES = {"continuations"}