Fix impossible unsigned integer comparisons in Abs() function

[Copilot]

Issue

The Abs() function in builtin/lib.go contained unreachable conditional branches for unsigned integer types. Unsigned integers cannot be negative, yet the code checked if x < 0 before returning.

case uint:
    if x < 0 {      // Always false - uint cannot be < 0
        return -x   // Unreachable
    } else {
        return x
    }

This pattern was repeated for uint, uint8, uint16, uint32, and uint64.

Changes

• builtin/lib.go: Removed impossible conditional checks for all unsigned integer types, simplified to direct return
• builtin/builtin_test.go: Added test coverage for all unsigned integer types including edge case of zero values

Result

case uint:
    return x
case uint8:
    return x
// etc.

Eliminates 20 lines of dead code, improves clarity, and removes a potential source of confusion for maintainers.

Original prompt

Find bugs

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