fix(content): guard optional durationMs and cover loop validation

src/engine/game.ts

@@ -169,6 +169,7 @@ export function tickGame(state: GameState, content: Content, tickMs: number): Ti
     const actionId = state.activeActionId;
     const action = content.actionsById[actionId];
     if (!action) return { completedActionIds };
+    if (action.durationMs === undefined) return { completedActionIds };
     if (state.actionElapsedMs < action.durationMs) return { completedActionIds };
 
     state.actionElapsedMs -= action.durationMs;