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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>