This PR fixes issue #9681 by replacing ConcurrentHashMap.newKeySet with Collections.synchronizedSet.

Problem

Scala Native ConcurrentHashMap has a race condition causing NPE when forking 10K fibers.

Solution

Use synchronized HashSet instead of ConcurrentHashMap.newKeySet in Scala Native.

Testing

• All 22 PromiseSpec tests pass on Scala Native
• Compilation successful

Closes #9681 /claim #9681