Event buffering during hydration gap

Clicks on island elements before hydration completes are captured
and replayed after boot finishes:

- shell.sx: inline script (capture phase) buffers clicks on
  [data-sx-island] elements that aren't hydrated yet into window._sxQ
- boot.sx: after hydration + process-elements, replays buffered clicks
  by calling target.click() on elements still connected to the DOM

This makes SSR islands feel interactive immediately — the user can
click a button while the SX kernel is still loading/hydrating, and
the action fires as soon as the handler is wired up.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/sx/templates/shell.sx

@@ -76,6 +76,8 @@ details.group{overflow:hidden}details.group>summary{list-style:none}details.grou
       (body :class "bg-stone-50 text-stone-900"
         ;; Server-rendered HTML — visible immediately before JS loads
         (div :id "sx-root" (raw! (or body-html "")))
+        ;; Event buffer — captures clicks during hydration gap, replayed after boot
+        (script (raw! "document.addEventListener('click',function(e){var i=e.target.closest('[data-sx-island]');if(i&&!i._sxBoundislandHydrated){e.stopPropagation();(window._sxQ=window._sxQ||[]).push({t:e.target,ts:Date.now()})}},true)"))
         (script :type "text/sx" :data-components true :data-hash component-hash
           (raw! (or component-defs "")))
         (when init-sx