Fix PoolAllocator crash on first allocation

[Tidenflow]

Summary

• Initialize StackLinkedList::head to nullptr.
• Return nullptr when StackLinkedList::pop() is called on an empty list.
• Return a successful exit code after all benchmarks complete.

Problem

After the PoolAllocator initialization was changed to use offset-based lazy allocation, its free list can be empty during the first allocation.

PoolAllocator::Allocate() expects pop() to return nullptr in that case, but StackLinkedList::pop() dereferences head unconditionally. This causes an access violation before the allocator can fall back to allocating a new chunk using the offset.

Verification

Built and ran the project with MinGW on Windows:

cmake -S . -B build -G "MinGW Makefiles"
cmake --build build
build\main.exe