Instantiate template classes

Instantiate our template EntityPool for the gameplay module. Create a header GameEntities.h that declares using BulletPool = EntityPool<Bullet, 1024>; extern BulletPool g_bullets; and ensure GameEntities.cpp defines BulletPool g_bullets; so the pool is shared across translation units. Confirm Bullet type is forward-declared in the header and the full include is only in the cpp. Ladder":"L1"

Before I expose g_bullets, make it easy for reviewers to see ownership and lifetime. Surface why we chose a global instance over passing references, document thread-safety expectations, explain why 1024 was chosen, and flag what must change to turn it into a dependency-injected pool for unit tests. Ladder":"L2"

Tests that create and destroy worlds report use-after-free during shutdown. I suspect global g_bullets outlives some systems or the Bullet type has non-trivial destructors that run in the wrong order. I can’t tell which subsystem’s static destructors run earlier. Should I convert the global instance to a function-local static, or change teardown ordering? Which is the safer first change to stabilize tests? Ladder":"L5"

Many modules create global instantiated template pools and we get intermittent test flakes and shutdown order problems. What consistent pattern should we adopt to manage lifetime and testability of instantiated template globals, and what small migration steps will make the code safer with minimal churn? Ladder":"L6"