Observer

MobX reactions + typed event emitters.

Where: MobX autorun/reaction for continuous state; pixi-reels' EventEmitter<ReelSetEvents> for discrete domain events.

The rule:

• Continuous state → MobX reaction.
• Discrete event ("reel landed") → event emitter.

Both are observer patterns; pick the primitive that fits the shape of the data. Reactions that fire every frame while reels spin are the wrong shape for "reel landed."