add lock-free stack (Treiber)

[AneesPatel]

What

Adds a lock-free stack using the Treiber algorithm. Push and pop both use compare_exchange_weak on the head pointer - no mutexes, no locks.

Notes

• ABA problem is documented in a comment. Not a concern for current use cases but would need tagged pointers or hazard pointers for production.
• Destructor drains the stack to free heap-allocated nodes.
• Stress test: 4 producers push 10k items each, 4 consumers race to drain, verify total count.

Files

• include/LockFreeStack.h
• tests/test_lockfreestack.cpp
• CMakeLists.txt - add test executable