Nonexclusive write lock via read-count bit overflow

Wrap-around in Concurrent::ReentrantReadWriteLock in ruby-concurrency concurrent-ruby before 1.3.7 allows local users to bypass write exclusivity. The per-thread held count shares its low 15 read-count bits with the WRITE_LOCK_HELD bit, so 32,768 reentrant reads make try_write_lock return true without setting RUNNING_WRITER. The calling thread needs access to the lock object and extreme reentrant read depth; concurrent readers can then race with supposedly exclusive writes and observe inconsistent protected state.