Sudoku Solver

Each empty cell starts with all N candidate digits. Row, column, and box constraints prune impossible values to a fixed point — naked singles, hidden singles, locked candidates, naked pairs, X-wing all fall out of the same propagation operator at different scopes. No WalkSAT, no annealing, no discrete flips. When propagation plateaus we branch on the most-constrained cell and recurse with bounded depth.