Add documented CRC library to /scapy/libs/

[Copilot]

Adds the CRC (Cyclic Redundancy Check) library from PR secdev#4855 to /scapy/libs/ with comprehensive documentation.

Documentation Added

• Module-level docstring: CRC fundamentals, parameter descriptions, usage patterns, references
• CRCParam class: Parameter container with signature generation and equality semantics
• _CRC_metaclass: Automatic table pre-computation, registry management, factory methods
• CRC base class: Single-shot and incremental (init/update/finish) APIs
• Pre-defined algorithms: CRC-16, CRC-32, CRC-16-CCITT, CRC-32-AUTOSAR with parameters and test vectors

Code Quality

• Consistent zero-padding in signature format
• Descriptive variable naming (data_len vs l)
• Proper format specifiers for hex output with 0x prefix

Usage

from scapy.libs.crc import CRC_32, CRC

# Single-shot computation
checksum = CRC_32(b"123456789")  # 0xcbf43926

# Incremental computation
crc = CRC_32.create_context()
crc.init()
crc.update(b"12345")
crc.update(b"6789")
checksum = crc.finish()  # 0xcbf43926

# Custom CRC variant
CustomCRC = CRC.from_parameters(
    poly=0x1021, size=16, init_crc=0xFFFF, xor=0,
    reflect_input=False, reflect_output=False
)

All test vectors pass. CodeQL reports 0 alerts.

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