Frozen BoxList instances cannot be pickled #344

A direct frozen BoxList cannot be pickled on develop (8364ce582dd4b4ff0d753e3efffa4b8675e809bc). The error occurs with every pickle protocol, including an empty list:

import pickle
from box import BoxList
value = BoxList([1, 2], frozen_box=True)
pickle.dumps(value)
# PicklingError / AttributeError:
# Can't pickle local object 'BoxList.__init__.<locals>.frozen'

BoxList.__init__ installs a local function on the instance to reject mutating methods. Pickle serializes those instance attributes, but cannot reference a locally defined function. The guard does not capture any local state, so using a module-level guard would make it serializable while retaining the same mutation errors after loading.

Expected: pickle round-trips preserve the values, box_options, and the existing frozen mutation guards. Ordinary mutable lists should continue to round-trip and remain mutable.

This is separate from #260 / PR #322 (nested Box options lost during unpickling) and #308 / PR #309 (nested Box subclass selection). PR #58 addressed older Python 3.7 pickling and outer-list freezing, but the local-function failure remains on current develop. No existing issue/PR for this specific failure was found in the full PR history or frozen/BoxList pickle searches.

Reproduced with the pure-Python implementation on CPython 3.14.6 / macOS. The Cython implementation also needs validation before submitting a fix. The contribution target is develop as specified in CONTRIBUTING.rst.