Fix LCM overflow handling and README GCD/LCM descriptions

[amartinsmg]

Motivation

• Prevent incorrect behavior when computing lcm due to intermediate int overflow and ensure correct absolute-value handling for long long results.
• Correct the documentation where the gcd.h and lcm.h feature descriptions were accidentally swapped in the README.md.

Description

• Updated src/lcm.h to cast operands to long long before multiplication and compute the absolute value with llabs to avoid intermediate overflow and match the return type.
• Replaced #include <math.h> with #include <stdlib.h> in src/lcm.h so llabs is available.
• Fixed the swapped feature descriptions for gcd.h and lcm.h in README.md so each header maps to the correct description.

Testing

• Ran make test which built the test executable successfully.
• Executed ./debug/test and all tests passed successfully.

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Codex Task