chore: import upstream snapshot with attribution

apps/sim/app/(landing)/hooks/use-lazy-mount.ts

@@ -0,0 +1,37 @@
+'use client'
+
+import { useEffect, useRef, useState } from 'react'
+
+/**
+ * Gates mounting a heavy client island on viewport proximity via
+ * {@link IntersectionObserver}, so its bundle (and any `requestAnimationFrame`
+ * loop) only loads once the section nears the viewport instead of on initial
+ * page load. Falls back to an eager mount when `IntersectionObserver` is
+ * unavailable, so the section never gets stuck unmounted.
+ *
+ * Pair with `next/dynamic(..., { ssr: false })` for the mounted component.
+ */
+export function useLazyMount(rootMargin: string) {
+  const ref = useRef<HTMLDivElement>(null)
+  const [inView, setInView] = useState(false)
+
+  useEffect(() => {
+    if (inView) return
+    if (typeof IntersectionObserver === 'undefined') {
+      setInView(true)
+      return
+    }
+    const el = ref.current
+    if (!el) return
+    const observer = new IntersectionObserver(
+      ([entry]) => {
+        if (entry.isIntersecting) setInView(true)
+      },
+      { rootMargin }
+    )
+    observer.observe(el)
+    return () => observer.disconnect()
+  }, [inView, rootMargin])
+
+  return { ref, inView }
+}