refactor: Rework Trainer generation

src/@types/session-data.ts

@@ -7,7 +7,7 @@ import type { ArenaData } from "#system/arena-data";
 import type { ChallengeData } from "#system/challenge-data";
 import type { ModifierData } from "#system/modifier-data";
 import type { PokemonData } from "#system/pokemon-data";
-import type { TrainerData } from "#system/trainer-data";
+import type { TrainerSaveDataSet } from "#system/trainer-save-data";
 
 /**
  * Save data for a run, as defined server-side
@@ -27,7 +27,7 @@ export interface SessionSaveData {
   waveIndex: number;
   battleType: BattleType;
   /** Only defined when the current wave is a trainer battle */
-  trainer: TrainerData | null;
+  enemyTrainers?: TrainerSaveDataSet;
   gameVersion: string;
   timestamp: number;
   challenges: ChallengeData[];

src/@types/utility-types.ts

@@ -70,6 +70,9 @@ export type Mutable<T> = {
 /** Requires an array to contain at least one item. */
 export type NonEmptyArray<T> = [T, ...T[]];
 
+/** A value that can be coerced into an array. */
+export type CoercibleArray<T> = T | T[];
+
 /**
  * Type helper to obtain the keys associated with a given value inside an object. \
  * Acts similar to {@linkcode Pick}, except checking the object's values instead of its keys.