Add three ECS systems for NPC simulation:
- needsDecaySystem: decrements hunger/energy each tick, clamped at 0
- npcBrainSystem: goal selection based on need thresholds with pathfinding
- movementSystem: advances entities along computed paths
Includes 8 passing tests covering decay, goal prioritization, and movement.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
