Fix infinite scroll and all sx-trigger/sx-get element binding

Root cause: process-elements during WASM boot-init marks elements as
processed but process-one silently fails (effect functions don't execute
in WASM boot context). Deferred process-elements then skips them.

Fixes:
- boot-init: defer process-elements via set-timeout 0
- hydrate-island: defer process-elements via set-timeout 0
- process-elements: move mark-processed! after process-one so failed
  boot-context calls don't poison the flag
- observe-intersection: native JS platform function (K.registerNative)
  to avoid bytecode callback issue with IntersectionObserver
- Remove SX observe-intersection from boot-helpers.sx (was overriding
  the working native version)

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

hosts/ocaml/browser/sx-platform.js

@@ -166,6 +166,22 @@
     document.cookie = args[0] + "=" + encodeURIComponent(args[1] || "") + ";path=/;max-age=31536000;SameSite=Lax";
   });
 
+  // IntersectionObserver — native JS to avoid bytecode callback issues
+  K.registerNative("observe-intersection", function(args) {
+    var el = args[0], callback = args[1], once = args[2], delay = args[3];
+    var obs = new IntersectionObserver(function(entries) {
+      for (var i = 0; i < entries.length; i++) {
+        if (entries[i].isIntersecting) {
+          var d = (delay && delay !== null) ? delay : 0;
+          setTimeout(function() { K.callFn(callback, []); }, d);
+          if (once) obs.unobserve(el);
+        }
+      }
+    });
+    obs.observe(el);
+    return obs;
+  });
+
   // ================================================================
   // Load SX web libraries and adapters
   // ================================================================