[Fire162]

Summary

A frozen BoxList (frozen_box=True) raised an AttributeError / PicklingError on pickle.dumps because the mutation guard was defined as a local function inside BoxList.__init__. Pickle serializes instance attributes but cannot serialize <locals>.frozen.

This moves the mutation guard function to module scope as _frozen_box_list_method. Frozen BoxList instances now round-trip cleanly through pickle across all pickle protocols while maintaining their frozen protection after deserialization.

Changes

• Defined module-level _frozen_box_list_method in box/box_list.py and referenced it in BoxList.__init__.
• Added test_frozen_list_pickle and test_box_list_pickle in test/test_box_list.py to verify pickling and mutation guards across all pickle protocols (for both populated and empty lists).
• Updated CHANGES.rst and AUTHORS.rst.

Fixes #344