refactor(simulation): isolate per-scene world state

[czlin7]

Summary

• introduce SimulationWorld as the owner of entity, component, and system managers
• move simulation system registration and update orchestration out of SimulationEngine / Scene_Simulation
• make EntityManager dependencies explicit non-null references
• remove unused per-frame spatial-hash work and stale ECS dependencies from the engine/scene layers

Why

This separates application infrastructure from simulation state so each simulation scene owns an isolated world instead of sharing mutable ECS state through SimulationEngine.

Validation

• local Debug build completed successfully
• foundation tests passed
• application smoke-tested through menu, simulation load, entity movement, return to menu, and reopening simulation