Mark elevator-opening-system as client

The file imports useEffect/useRef from React, which Next.js RSC builds
flag as client-only. Other core systems (e.g. elevator-runtime-system)
use useFrame from @react-three/fiber and slip through, but this one
needs the directive explicitly.

Fixes Turbopack build failure in private-editor community app:
"You're importing a module that depends on useEffect into a React
Server Component module."

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/src/systems/elevator/elevator-opening-system.tsx

@@ -1,3 +1,5 @@
+'use client'
+
 import { useEffect, useRef } from 'react'
 import type { AnyNode } from '../../schema'
 import useScene from '../../store/use-scene'