- Child tracebacks survive Future.result(): the worker carries its
  rendered traceback on the failure and the parent re-attaches it via
  __cause__, so result() shows the originating frames.  An unpicklable
  exception becomes a RuntimeError preserving that traceback rather than
  degrading silently into SubmissionDied.

- BaseException escaping a worker is now diagnosable: a SystemExit or
  KeyboardInterrupt raised by fn is wrapped as SubmissionDied with the
  cause chained, while a causeless SubmissionDied still means a truly
  silent death (SIGKILL, os._exit, failed send).

- CallbackRaised reports a registration seqno: Future.when_done() returns
  a per-Future seqno (user callbacks start at 0) carried on
  CallbackRaised.seqno, so a caught exception ties back to the registering
  call.

- concurrent.futures.Future.cancel() is wired through JobserverExecutor:
  cancelling a PENDING future tells the dispatcher to prune the work before
  dispatch rather than running it and discarding the result, via a weakref
  callback that no longer pins the executor alive.

- map() reclaims slots more eagerly: abandoning the returned generator
  (close()/del/GC or a Blocked deadline) reclaims finished workers' slots,
  and children blocked mid-send on a large result (exceeding the ~64KB pipe
  buffer) are drained via their result connection.

- JobserverExecutor fails loudly instead of hanging: the receiver loop
  marks the executor broken and fails all outstanding futures with
  BrokenExecutor (chaining the cause), catches a cancel/Started
  InvalidStateError race, and cleans up an owned Jobserver on startup
  failure.

- submit()/shutdown() race fixed: a submit racing shutdown(cancel_futures=
  True) could land after the blanket Cancel and still run; the dispatcher
  now latches a cancelling flag so any late Submit is cancelled on arrival.

- MinimalQueue hardened: put() is narrowed to a single item to remove a
  partial-sequence hazard on BrokenPipeError, get()/put() read the
  connection under the lock to close a close-vs-use race, and a plain
  threading.Lock replaces the cross-process Lock.

- Earlier and clearer validation: env keys/values must be strings, map()'s
  argses elements must be iterable with eager length-mismatch reporting,
  non-dict Mapping kwargs (e.g. ChainMap) are coerced to dict, and an
  unpicklable preexec_fn is detected before launch under spawn/forkserver.

- Resource leaks surfaced: Future.__del__ emits a ResourceWarning for an
  abandoned future still holding a connection, and an orphaned future is
  cleaned up when put() fails for any reason, including BaseException.

- API surface trimmed: MinimalQueue, resolve_context, and the
  deadline/timeout helpers are removed from the public __all__, and
  absolute_deadline/relative_timeout are renamed timeout_to_deadline/
  deadline_to_timeout.

- Many documentation clarifications: Jobserver thread-safety contract,
  picklability under spawn/forkserver, the shutdown(wait=False) caveat,
  orphaned-worker risk, map() chunksize memory behavior, and
  ignore_sigpipe scope.