[Core]: Implement entities manager

[nircoe]

• Implement entities central manager with type-indexed component_pool storage
• Refactor component_pool to inherit basic_component_pool abstract base for type erasure
• Add erase_at(dense_index) to sparse_set for index-based removal (eliminates double lookup in remove())
• Change component_pool::get() to return T& with assert (fail-fast over std::optional)
• Remove entity::valid() (liveness check belongs to entities::valid())
• Add EntitiesTests suite covering lifecycle, components, for_each, and bulk performance
• Consolidate integration tests into entities_tests.cpp, remove redundant test files