feat(solvers): implement solution iterator for RecursiveBacktrackingSolver (#60) - #111

Summary of Changes

This pull request implements lazy solution iteration via getSolutionIter() for RecursiveBacktrackingSolver, resolving #60 and bringing it to full feature parity with BacktrackingSolver and OptimizedBacktrackingSolver. (Supersedes closed #110 which was closed when the fork was accidentally cleaned up).

Key Implementation Details

1. Generator Method (recursiveBacktrackingIter):
   • Added recursiveBacktrackingIter(self, domains, vconstraints, assignments) generator to yield solutions on demand using recursive depth-first backtracking with Degree and MRV heuristics.
   • Wrapped constraint checking and domain state unwinding in a try...finally block to ensure domain.popState() is reliably executed if the generator is closed early (e.g. next(problem.getSolutionIter())).
2. Solver Interface:
   • Implemented RecursiveBacktrackingSolver.getSolutionIter() to return the lazy iterator.
3. Documentation & Doctests:
   • Updated class docstring and doctests in RecursiveBacktrackingSolver to demonstrate and verify getSolutionIter().
4. Unit Tests:
   • Added test_recursive_backtracking_solver_iterator in tests/test_solvers.py verifying lazy yielding, early stopping, unsatisfiable problems, and solution equivalence with BacktrackingSolver across both forwardcheck=True and forwardcheck=False.

Fixes #60.