/claim #9681

Proposed changes

This PR hardens the Scala Native concurrent-set path used by WeakConcurrentBag during fork-heavy workloads:

• Native PlatformSpecific.newConcurrentSet now uses:
  • Collections.newSetFromMap(new ConcurrentHashMap(...))
• It avoids the ConcurrentHashMap.newKeySet(...).add (KeySetView) code path seen in #9681 stack traces.

I also added regression-oriented native stress coverage:

• core-tests/shared/src/test/scala/zio/internal/ConcurrentSetSpec.scala
  • concurrent insert stress with colliding hash codes (nonFlaky(25), nativeOnly)
• PromiseSpec
  • waiter stack safety under sustained forking pressure

Why this shape

A first attempt using synchronized HashSet avoided the NPE path but failed concurrent correctness under stress (lost inserts). This version keeps true concurrent-map backing while avoiding newKeySet.

Proof

Commands executed locally:

SBT_OPTS='-Xms1G -Xmx3G -Xss4M -XX:+UseG1GC' sbt \
  "coreTestsNative/testOnly zio.internal.ConcurrentSetSpec -- -t \"handles colliding hash codes under concurrent inserts\"" \
  "coreTestsNative/testOnly zio.PromiseSpec -- -t \"waiter stack safety\""

Results:

• ConcurrentSetSpec passed (25 repeated runs)
• PromiseSpec waiter stack-safety tests passed