Add XTEA block cipher implementation

[PentesterTN]

Implemented the XTEA block cipher from scratch. It's a lightweight 64-bit Feistel cipher that's still used in embedded systems and game protocols. Includes encrypt/decrypt with full doctest coverage and input validation.

• 64-bit block size, 128-bit key, 64 rounds (Feistel network)
• Proper uint32 wrapping throughout
• Only uses the struct module from stdlib

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

[algorithms-keeper]

Closing this pull request as invalid

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