complex: Fix expm1 formula computing exp(z-1) instead of exp(z)-1

[duncanita]

Summary

• Fix mathematical error in complex expm1 function
• Add tests for complex expm1 to prevent regression

Problem

The complex implementation of expm1(z) incorrectly computed e^(z-1) instead of e^z - 1.

For example:

• expm1(0) returned e^(-1) ≈ 0.3679 instead of 0
• expm1(1) returned e^0 = 1 instead of e - 1 ≈ 1.7183

Fix

Changed src/complex.cc line 1275 from exp(z - one) to exp(z) - one.

Known Limitations

This fix corrects the formula but is still a temporary solution. For small values of z, computing exp(z) - 1 directly suffers from catastrophic cancellation. A proper implementation should use a numerically stable algorithm (e.g., Taylor series for small |z|).

Testing

• Added 3 new tests for complex expm1 in complex_functions()
• Ran full test suite with -T flag - no regressions