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/static/wasm/sx/boot.sx

@@ -492,7 +492,23 @@
       (process-elements nil)
       ;; Wire up popstate for back/forward navigation
       (dom-listen (dom-window) "popstate"
-        (fn (e) (handle-popstate 0))))))
+        (fn (e) (handle-popstate 0)))
+      ;; Replay buffered clicks from hydration gap
+      (let ((queued (host-get (dom-window) "_sxQ")))
+        (when queued
+          (let ((arr (host-call (host-global "Array") "from" queued)))
+            (let ((n (host-get arr "length")))
+              (when (> n 0)
+                (log-info (str "replaying " n " buffered click(s)"))
+                (let loop ((i 0))
+                  (when (< i n)
+                    (let ((entry (host-call arr "at" i)))
+                      (when entry
+                        (let ((target (host-get entry "t")))
+                          (when (and target (host-get target "isConnected"))
+                            (host-call target "click")))))
+                    (loop (+ i 1)))))))
+          (host-set! (dom-window) "_sxQ" nil))))))
 
 
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