GeezerVison

…0253). (pythonGH-31976)

(cherry picked from commit cd44afc)
(cherry picked from commit a5b7678)

Co-authored-by: Serhiy Storchaka <[email protected]>

…H-31976) (GH-31981)

(cherry picked from commit cd44afc)
(cherry picked from commit a5b7678)

). (pythonGH-31976) (pythonGH-31981)

(cherry picked from commit cd44afc)
(cherry picked from commit a5b7678)

Replaces BufferedWriter's bytearray + _write_lock with a _Nibbler buffer
manager built around _Morsel work units. Adds FileIO._writev so the
nibbler can issue a single writev(2) for multi-buffer flushes when
available, falling back to coalesce+write otherwise.

The Nibbler holds its lock end-to-end across raw I/O, matching the
pre-existing cross-thread write ordering guarantee. A future two-lock
split (short buffer lock + I/O lock) is sketched in the class docstring
as the intended path for releasing the I/O lock once profiling justifies
it.

Also restores f.close() in BufferSizeTest.try_one's nested finally and
skips test_slow_close_from_thread pending a way to validate pythongh-76157 /
pythongh-31976 that does not assume the old lock-across-I/O behavior.

See: https://discuss.python.org/t/reworking-buffered-i-o-in-cpython/90019

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>