Default to loky executor on all platforms (fixes forkserver breakage on Python 3.14)

[basnijholt]

What

Make loky.get_reusable_executor the default executor on all platforms, instead of only macOS/Windows. Removes the Linux-only concurrent.futures.ProcessPoolExecutor default and the now-dead lambda-pickling check in AsyncRunner that only applied to it. Updates the parallelism tutorial accordingly.

Why

Python 3.14 changed the default multiprocessing start method on Linux from fork to forkserver (python/cpython#84559). Forkserver workers re-import __main__ instead of inheriting the parent's memory, so functions defined interactively (notebooks, executed doc pages) — which pickle stores by reference as __main__.<name> — fail to unpickle in the workers:

File ".../python3.14/multiprocessing/queues.py", line 120, in get
    return _ForkingPickler.loads(res)
AttributeError: module '__main__' has no attribute 'sphere'

(e.g. when running docs/source/tutorial/tutorial.LearnerND.md on Linux + Python 3.14.)

The old comment in runner.py justified the Linux fast-path with "On Linux the whole process is forked, so the issue does not appear" — that assumption no longer holds. loky serializes functions by value via cloudpickle, so it works everywhere, and it is already a required dependency (loky >= 2.9 in core dependencies, imported unconditionally in runner.py).

Testing

• Full suite passes locally on Python 3.14.5 (macOS), except 3 pre-existing test_to_dataframe failures (KeyError: -1) that also fail on unmodified main under 3.14 — unrelated to this change, likely a pandas-on-3.14 issue; will open a separate issue.
• CI nox matrix already includes 3.14, so this PR's CI exercises the new default there.
• pre-commit (ruff, mypy, etc.) passes.