Fix SBX prob_bin parameter to control parent exchange probability

[blankjul]

Summary

This PR fixes the prob_bin parameter in the SBX (Simulated Binary Crossover) implementation to properly control the probability of parent value exchange.

Problem

The current implementation has a semantic issue where prob_bin controls random swapping based on value magnitude rather than parent identity. This creates inconsistent behavior depending on which parent has larger/smaller values.

Solution

• Preserve parent identity during SBX crossover calculations
• Make prob_bin represent the probability that parents exchange values
• Ensure Child 1 inherits characteristics from Parent 1 when prob_bin=0.0
• Use cleaner np.where syntax for child assignment and exchange

Behavior

With this fix:

• prob_bin=0.0: Child1←Parent1, Child2←Parent2 (no exchange)
• prob_bin=0.25: 25% chance of exchange (Child1←Parent2, Child2←Parent1)
• prob_bin=0.5: 50% chance of exchange (equivalent to old behavior)
• prob_bin=1.0: Always exchange (Child1←Parent2, Child2←Parent1)

Backward Compatibility

The fix maintains equivalent behavior at prob_bin=0.5 (the default), ensuring backward compatibility while providing the correct semantics at other values.

Closes #673

[lukepmccombs]

I have checked the code and ran the test experiment from my PR; this commit successfully fixes the bias and performance discrepancy. Looks good to me!