add hazard pointers for safe memory reclamation

[AneesPatel]

What

Hazard pointer implementation for deferred memory reclamation. Fixes the use-after-free problem that's lurking in the Treiber stack (PR #1).

The problem

In a lock-free pop: thread A reads head, gets preempted. Thread B pops the same node and deletes it. Thread A resumes and dereferences a freed pointer. Bad.

The fix

Before dereferencing any pointer that might be concurrently freed, publish it in a per-thread hazard slot. When retiring a node, scan all published hazard pointers first - only free it if nobody has it published.

Notes

• Global hazard table: MAX_THREADS=16, one slot per thread (thread-local, acquired on first use).
• Retire list is scanned lazily every BATCH_SIZE=32 retirements - not on every pop.
• Test pairs hazard pointers with a SafeStack variant, verifies correct pop count across 4 prod + 4 cons threads.
• This is the simplest version that actually works. Production code would use dynamic slot counts and smarter scan heuristics.

Files

• include/HazardPointers.h
• tests/test_hazardpointers.cpp
• CMakeLists.txt