Add CRC-32 checksum implementation

[PentesterTN]

Implemented CRC-32 from scratch using the standard reflected polynomial (0xEDB88320) with a precomputed 256-entry lookup table. This is the same algorithm used in ZIP, PNG, gzip, and Ethernet frames.

• No external dependencies -- pure Python lookup table approach
• Doctests verified against zlib.crc32() to make sure the output is correct
• Handles empty input and arbitrary binary data

Reference: https://en.wikipedia.org/wiki/Cyclic_redundancy_check

[algorithms-keeper]

Closing this pull request as invalid

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