Add spatial hash self-collision to Verlet cloth

[mmmovania]

Summary

• add spatial hashing based self-collision detection to the Verlet cloth example
• expose tunable collision radius and cell sizing derived from cloth resolution
• fix the IntegrateVerlet function so the simulation loop closes cleanly and clamps particles above the floor

Testing

• g++ -std=c++11 -c OpenCloth_Verlet/OpenCloth_Verlet/main.cpp (fails: missing GL/glew headers in container environment)

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Codex Task