feat: optimize powers computation in permutation functions

[berZKerk]

Motivation

The current implementation uses random_elements[1].powers() iterator that computes powers on-the-fly, which is inefficient in inner loops. There's a TODO comment explicitly requesting this optimization.

Solution

Pre-compute beta powers (up to 16) and store in vector instead of using iterator. This avoids repeated computation in loops while maintaining same functionality.

Changes:

• populate_local_permutation_row: Replace powers() iterator with pre-computed vector
• eval_permutation_constraints: Apply same optimization
• Add explanatory comments

PR Checklist

• Added Tests (existing tests pass)
• Added Documentation (no documentation changes needed)
• Breaking changes (no breaking changes)