Prevent concurrent fragment resolution from corrupting directive stacks

[Copilot]

Concurrent object resolvers shared mutable fragment directive stacks, allowing fragment traversal on separate threads to race during push_front/pop_front and cause double-free or segmentation faults.

• Resolution isolation

  • Snapshot inherited fragment definition, spread, and inline-fragment stacks for each selection visitor.
  • Preserve inherited directive context without sharing mutable list nodes across asynchronous resolvers.

• Regression coverage

  • Exercise the same fragment beneath three concurrently resolved fields in the threaded coroutine test.

• Fixes Using fragments in query sometimes results in double-free/SEGV sanitizer crashes. #357