Finish the opening milestone's switching, items and acceptance run

- Decode evolution lists from the ROM; all 251 match the pinned PRET source.
- Compare CalcMonStats and CalcExpAtLevel against the original routines in
  the reference emulator.
- Add a continuous opening playthrough from the intro to the catching
  tutorial, reloading a JSON save after every stage.
- Port player switching in battle: the PKMN menu, forced choices after a
  faint, Shift mode's offer and double faints, with the original texts.
- Port HP and status healing items in battle through the ROM's ItemEffects,
  HealingHPAmounts and StatusHealingActions tables.
- Add the battle party menu and the Items pocket to the UI.

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

tests/battle-fixture.ts

@@ -2,7 +2,7 @@ import { CrystalAssets } from "../src/game/rom/assets";
 import { generatePartyMon } from "../src/game/party/pokemon";
 import { GameWorld } from "../src/game/world/world";
 
-export type BattleScenario = "win" | "loss" | "unsupported" | "catch";
+export type BattleScenario = "win" | "loss" | "unsupported" | "catch" | "switch" | "potion";
 
 /** Test-only encounters on an existing floor: production maps stay gated.
  * Everything after selection runs through the ROM's real wild-battle script. */
@@ -28,7 +28,16 @@ export function battleFixtureWorld(assets: CrystalAssets, scenario: BattleScenar
     mon.pp = [10, 0, 0, 0];
   }
   world.state.party.push(mon);
+  if (scenario === "switch")
+    world.state.party.push(generatePartyMon(assets.pokemon, {
+      species: 158, level: 5, item: 0, otId: world.playerId,
+      ot: "CHRIS", dvs: 0x823b, caughtData: 0,
+    }));
   if (scenario === "catch") world.state.bag.balls = [{ item: 5, quantity: 10 }]; // POKé BALL
+  if (scenario === "potion") {
+    mon.hp = 3;
+    world.state.bag.items = [{ item: 18, quantity: 1 }]; // POTION
+  }
   world.state.pokedex.seen.add(155);
   world.state.pokedex.caught.add(155);
   world.state.money = 3001;