feat: add crossTopTwo and crossTopThree multi-parent crossover operators

[Yadelew-zemene]

Purpose

Introduce multi-parent crossover operators to improve exploration in hill climbing:

• crossTopTwo → pairwise crossover
• crossTopThree → 3-simplex crossover

What’s included

• Added crossTopTwo with pairwise instance merging
• Added crossTopThree with triple-instance merging
• Implemented structured loop hierarchy (inner / middle / outer)
• Added helper functions for instance extraction and merging
• Added unit tests for loop components and top-level functions

Key idea

Instead of crossing a single parent with others, this introduces:

• pairwise recombination (2-parent)
• triple recombination (3-parent)

This enables richer interaction between high-quality candidates and improves diversity of generated instances.

Notes for reviewers

Recommended review order:

1. cross-top-two-helpers.metta
2. cross-top-two.metta
3. cross-top-three-helpers.metta
4. cross-top-three.metta
5. test/cross-top-two-test.metta
6. test/cross-top-three-test.metta

Testing

• Unit tests added for:
  • merge helpers
  • loop structures (inner / middle / outer)
• Structural tests validate limit constraints on generated instances