Add simple C++ cubing demo

[piyush-J]

Summary

• implement a C++ example simple_mcts.cpp that mimics AlphaMapleSAT cubing
• include BCP-based unit propagation routines
• parse command line options matching the Python tool
• print summary lines for variables considered, free variables, runtime, and nodes
• document how to build and run the C++ demo in the README

Testing

• python3 -m py_compile alphamaplesat/main.py
• g++ -std=c++17 -O2 simple_mcts.cpp -o simple_mcts
• ./simple_mcts alphamaplesat/constraints_17_c_100000_2_2_0_final.simp -m 5 -o out.cubes -numMCTSSims 1