/claim #9681

ConcurrentHashMap.newKeySet() isn’t implemented in Scala Native’s JDK compatibility layer, which caused a NPE when WeakConcurrentBag tried to create its internal set during high-fiber-count scenarios. Replaced both overloads with Collections.newSetFromMap(new ConcurrentHashMap(...)), which achieves the same concurrent set semantics using APIs that Scala Native actually supports. I verified that the equivalent newWeakSet already used Collections.newSetFromMap pattern, so this is consistent with existing practice in the file.

Fixes #9681