Add 3x3 Sudoku — Grover's Algorithm notebook to games/

[y2sud]

Summary

This PR adds a new Jupyter notebook to the games/ folder demonstrating Grover's algorithm applied to a 3×3 Latin-square Sudoku puzzle.

• Notebook: games/3x3_sudoku_grover.ipynb
• One-liner: Solves a 3×3 Latin-square Sudoku using Grover's algorithm.

What the notebook covers

• Encodes 6 unknown Sudoku cells as 2-qubit variables (00→0, 01→1, 10→2), totalling 22 qubits (12 variable + 9 clause ancillae + 1 output)
• Builds a phase-kickback oracle that marks valid Latin-square solutions by checking all 15 row/column inequality constraints via neq_pair and neq_const gadgets (compute → kick → uncompute pattern)
• Prepares each cell in a restricted superposition (1/√3)(|00⟩+|01⟩+|10⟩) to exclude the invalid |11⟩ codeword
• Runs the optimal number of Grover iterations (k ≈ (π/4)·√(N/M), N=729, M=2) and measures the circuit on Qiskit Aer
• Displays results as a colour-coded histogram: green bars = valid solutions, blue = non-solutions

Changes

• games/3x3_sudoku_grover.ipynb — new notebook
• games/README.md — added entry for the new notebook under Contents

Dependencies

qiskit >= 1.0, qiskit-aer >= 0.14, matplotlib >= 3.7 — all standard Qiskit ecosystem packages.

Testing

Runs end-to-end on a standard laptop in under one minute using the Aer statevector simulator. Both valid solutions ([1,2,0,2,0,1] and [2,0,1,1,2,0]) are consistently the top-ranked measurements.

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