feat(crossover): implement legacy MOSES crossover operators

[Yadelew-zemene]

Description

This PR introduces multi-parent crossover capability from legacy MOSES into the MeTTa-based crossover system.
It restores and adapts crossTopTwo and crossTopThree to work with the current architecture, enabling stable multi-parent recombination in hill-climbing optimization.

Main Motivation

• Improve diversity and exploration in hill-climbing search

What this enables :

      -    Supports 2-parent and 3-parent crossover in the current system
      -    Restores missing legacy MOSES exploration behavior
      -    Improves search diversity during hill-climbing optimizations

Changes

     -      Built crossTopTwo (pairwise crossover)
     -      Built crossTopThree (triple crossover)
     -      Added helper utilities for instance handling and merging
     -      Integrated operators into existing crossover pipeline

Testing

     -     Unit tests for crossTopTwo, crossTopThree, and helpers
     -     Structural validation of crossover flow 
     -     End-to-end pipeline test on sample demes