Port the start menu, the party menu and the pack outside battle

Start opens the original start menu. POKéMON shows the party with the
Pokémon submenu (stats, switching, move order, held items); PACK shows the
four pockets with the field item effects for healing, PP and the Bicycle,
plus giving and tossing. The menu runs on a script draft, and its cursors
persist until a battle clears them. Select keeps the Key Items shortcut.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/game/battle/battle.ts

@@ -169,6 +169,8 @@ export interface BattleOptions {
   tutorial?: boolean;
   /** GetWorldMapLocation for the current map (LevelUpHappinessMod). */
   landmark?: number;
+  /** wPartyMenuCursor as the last party menu outside battle left it. */
+  partyMenuCursor?: number;
 }
 
 /** BATTLERESULT_BOX_FULL: a catch filled the current box. */
@@ -424,9 +426,9 @@ export class Battle {
   private battleHasJustStarted = 0;
   /** wCurPartyMon: the party index a menu picked. */
   private curPartyMon = 0;
-  /** wPartyMenuCursor: the 1-based cursor of the last party menu, which
-   * no ported field menu sets before a battle. */
-  private partyMenuCursor = 0;
+  /** wPartyMenuCursor: the 1-based cursor of the last party menu, from
+   * the start menu's until the battle's own party menu sets it. */
+  private partyMenuCursor: number;
   /** wLastPlayerMon: the Pokémon that was out before the last switch. */
   lastPlayerMon = 0;
   /** wEnemyIsSwitching: ParseEnemyAction keeps the current move. */
@@ -455,6 +457,7 @@ export class Battle {
   constructor(private options: BattleOptions) {
     this.data = new BattleData(options.data.reader);
     this.matchups = this.data.typeMatchups();
+    this.partyMenuCursor = options.partyMenuCursor ?? 0;
     if (options.trainer) this.battleMode = C.TRAINER_BATTLE;
     if (options.tutorial) this.battleType = C.BATTLETYPE_TUTORIAL;
   }