Add generic codec base class combining shared ASN.1/CBOR codec logic

[Copilot]

BERcodec_Object and CBORcodec_Object duplicated the same metaclass registration pattern, check_string, dec (safe-mode skeleton), and safedec. This extracts those identical parts into a shared base in scapy/libs/codec.py.

New: scapy/libs/codec.py

• GenericCodec_metaclass — handles c.tag.register(c.codec, c) once; exposes _handle_registration_error hook for format-specific logging
• GenericCodecObject[_K] — provides the shared methods:
  • check_string — parameterised by _decoding_error_class
  • dec — parameterised by _generic_error_classes + _decoding_error_object_class; covers CBOR fully, overridable for BER's BadTag recovery
  • safedec — was 100% identical in both; now lives in one place
  • enc — abstract stub

scapy/asn1/ber.py

• BERcodec_metaclass → inherits GenericCodec_metaclass; overrides _handle_registration_error with warning()
• BERcodec_Object → inherits GenericCodecObject; removes check_string + safedec; retains dec override for BER_BadTag_Decoding_Error recovery path

scapy/cbor/cborcodec.py

• CBORcodec_metaclass → inherits GenericCodec_metaclass; overrides _handle_registration_error with log_runtime.error()
• CBORcodec_Object → inherits GenericCodecObject; removes check_string, dec, and safedec entirely — all three are now fully inherited

Usage pattern for new codec formats

from scapy.libs.codec import GenericCodec_metaclass, GenericCodecObject

class MyCodec_metaclass(GenericCodec_metaclass):
    @classmethod
    def _handle_registration_error(cls, c, exc):
        log_runtime.warning("Failed to register %r" % c)

class MyCodecObject(GenericCodecObject[_K], metaclass=MyCodec_metaclass):
    codec = My_Codecs.MY_FORMAT
    tag   = My_Tags.SOME_TAG
    _decoding_error_class        = MyDecodingError
    _generic_error_classes       = (MyDecodingError, MyError)
    _decoding_error_object_class = MY_DECODING_ERROR

    @classmethod
    def do_dec(cls, s, context=None, safe=False):
        ...  # format-specific logic only

    @classmethod
    def enc(cls, s):
        ...  # format-specific logic only

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