Fixing the implementation of uint128

[KelvinMYYZJ]

As discussed in #389, the AI-generated fallback implementation of uint128 contains several correctness issues.

This PR checks the implementation and fixes multiple problems:

• Rewrite the multiplication implementation, with detailed comments explaining its correctness.
• Fix signed integer comparison to correctly handle the sign bit.
• Fix signed integer division to conform to the C standard semantics.
• Fix right shift on signed integers to use arithmetic shifting, preserving the sign for negative values.

These changes improve the correctness and consistency of the fallback implementation, especially for signed arithmetic edge cases.