Add a Sudoku solver (backtracking algorithm)

[utk2103]

Add a Sudoku solver using backtracking to scripts/.

Why

scripts/ has linear search, binary search, merge sort, and Tower of Hanoi. Backtracking is the missing algorithm family, and Sudoku is the friendliest example of it — the recursion is visible in the grid.

Suggested file

scripts/sudoku_solver.py

What it should do

• Hold a 9×9 puzzle as a list of lists, 0 for blanks.
• Solve it in place: find the next empty cell, try 1–9, recurse, undo on failure.
• Print the grid before and after, in 3×3 blocks.

Acceptance criteria

• Recursive backtracking with an explicit undo step (grid[r][c] = 0) — not a brute-force permutation generator.
• is_valid(grid, row, col, value) checks row, column, and the 3×3 box.
• Returns False for an unsolvable puzzle instead of hanging or crashing.
• assert-based self-check on one known puzzle with a known solution.
• Standard library only.
• Short comment on why backtracking beats trying every filling.

Inspiration

sudoku-solver from Awesome-Python-Scripts.

Good first issue for anyone comfortable with recursion — comment below to get assigned.