- JobserverExecutor can own a default-constructed Jobserver: passing no
  argument creates and manages one automatically; shutdown(wait=True)
  closes it.  This makes JobserverExecutor very drop-in for other code.

- O(k) slot acquisition: _obtain_tokens() reuses a persistent
  DefaultSelector registered once per Jobserver rather than per call,
  reducing per-submission overhead at scale.

- Removed callbacks keyword from Jobserver.submit() because of above
  algorithmic improvement.

- preexec_fn may return a context manager: the child function runs inside
  it via ExitStack, enabling entry/exit semantics around each job.

- Jobserver.__exit__ warns on suppressed CallbackRaised: a RuntimeWarning
  is emitted when __exit__ silences a CallbackRaised.

- ResourceWarning on unclosed Jobserver: __del__ emits a ResourceWarning
  when an open instance is garbage-collected without being closed.

- Nesting examples generalised to all multiprocessing start methods:
  ex02_nested.py now covers fork, forkserver, and spawn.