Prevent clear and in-place operators from mutating frozen BoxLists

[vitalivo]

A BoxList([1, 2], frozen_box=True) can still be modified with clear(), += [3], or *= 2 (including *= 0). These operations bypass the frozen checks and can change the hash of an object already used as a dictionary key.

Block clear alongside the other frozen methods and check frozen_box in the two in-place special methods, which Python resolves on the class. Mutable lists retain their existing behavior. Four regression cases fail before the fix; they also check unchanged contents/hash after rejection and the corresponding mutable operations.

Validation: all 163 tests pass on Python 3.13 with the optional serialization dependencies, including toon-format. Includes changelog and contributor updates; targets develop as requested by CONTRIBUTING.rst.