feat: battle system, state manager enhancements, engine and demo updates

engine/core/Engine.ts

@@ -51,9 +51,9 @@ export class Engine {
     this.audioSystem = new AudioSystem()
     this.achievementSystem = new AchievementSystem()
 
-    this.stateManager.onAfterApply = (vars) => {
+    this.stateManager.onAfterApply.add((vars) => {
       this.achievementSystem.check(vars)
-    }
+    })
 
     this.videoManager.onTimeUpdate(this.onTimeUpdate)
   }

engine/core/StateManager.ts

@@ -4,7 +4,7 @@ export class StateManager {
   variables: Record<string, number> = {}
   flags: Set<string> = new Set()
   history: ChoiceRecord[] = []
-  onAfterApply: ((variables: Record<string, number>) => void) | null = null
+  onAfterApply: Set<((variables: Record<string, number>) => void)> = new Set()
 
   init(initialVars: Record<string, number>) {
     this.variables = { ...initialVars }
@@ -73,7 +73,7 @@ export class StateManager {
           break
       }
     }
-    this.onAfterApply?.(this.variables)
+    this.onAfterApply.forEach((cb) => cb(this.variables))
   }
 
   recordChoice(choice: ChoiceRecord) {