Port the Johto badge stat boosts (#31)

BadgeStatBoosts runs where the original calls it: when a Pokémon is sent out, after an in-battle level-up, and in CalcPlayerStats. It keeps the PlainBadge/MineralBadge swap and the Glacier Badge's Special Defense register bug. A transformed Pokémon that levels up now keeps its copied stats.

The reference emulator compares all 256 badge bytes over 28 stat sets per browser against the unmodified routine.

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

src/game/battle/battle.ts

@@ -908,7 +908,37 @@ export class Battle {
     };
     this.player.stats = mon.stats.slice(1);
     this.applyStatusEffects(this.player);
-    // No badges exist yet, so BadgeStatBoosts changes nothing.
+    this.badgeStatBoosts();
+  }
+  /** BadgeStatBoosts: Johto badges raise the battle Pokémon's Attack,
+   * Defense, Speed and Special Attack by an eighth (to at most 999). The
+   * PlainBadge and MineralBadge bits are swapped first. The Glacier Badge's
+   * Special Defense boost tests the register that BoostStat leaves after
+   * Special Attack, so it depends on that stat (a kept bug). */
+  private badgeStatBoosts() {
+    const badges = this.options.johtoBadges ?? 0;
+    // ZEPHYR, HIVE, FOG, STORM, GLACIER and RISING stay; PLAIN (bit 2)
+    // moves to bit 4 and MINERAL (bit 4) to bit 2.
+    let b = (badges & 0xeb) | ((badges & 0x04) << 2) | ((badges & 0x10) >> 2);
+    const stats = this.player.mon.stats;
+    // BoostStat, returning the A register it leaves.
+    const boost = (index: number) => {
+      const value = stats[index] + (stats[index] >> 3);
+      if (value >= C.MAX_STAT_VALUE) {
+        stats[index] = C.MAX_STAT_VALUE;
+        return C.MAX_STAT_VALUE & 0xff;
+      }
+      stats[index] = value;
+      return ((value >> 8) - (C.MAX_STAT_VALUE >> 8) - ((value & 0xff) < (C.MAX_STAT_VALUE & 0xff) ? 1 : 0)) & 0xff;
+    };
+    let a = 0;
+    for (let index = 0; index < 4; index++) {
+      a = b;
+      const carry = b & 1;
+      b >>= 2;
+      if (carry) a = boost(index);
+    }
+    if (a & 1) boost(4);
   }
   /** ApplyPrzEffectOnSpeed and ApplyBrnEffectOnAttack for one side. */
   private applyStatusEffects(side: Side) {
@@ -1599,6 +1629,7 @@ export class Battle {
       const value = Math.floor((player.stats[stat] * numerator) / denominator);
       player.mon.stats[stat] = value === 0 ? 1 : Math.min(999, value);
     }
+    this.badgeStatBoosts();
     this.applyStatusEffects(this.turn === 0 ? this.player : this.enemy);
   }
   /** PrintPartyMenuActionText with the Pokémon's nickname. */
@@ -4985,13 +5016,15 @@ export class Battle {
     }
     this.failedMessage = 0;
   }
-  /** CalcPlayerStats / CalcEnemyStats: stages, then status penalties. */
+  /** CalcPlayerStats / CalcEnemyStats: stages, the player's badge
+   * boosts, then status penalties. */
   private calcStats(side: Side) {
     for (let stat = 0; stat < 5; stat++) {
       const [numerator, denominator] = this.data.statMultiplier(side.levels[stat]);
       const value = Math.floor((side.stats[stat] * numerator) / denominator);
       side.mon.stats[stat] = value === 0 ? 1 : Math.min(999, value);
     }
+    if (side === this.player) this.badgeStatBoosts();
     this.applyStatusEffects(side);
   }
   private statName() {
@@ -6019,8 +6052,13 @@ export class Battle {
         side.mon.hp = mon.hp;
         side.mon.maxHp = mon.stats[0];
         side.mon.level = newLevel;
-        side.stats = mon.stats.slice(1);
-        this.calcStats(side);
+        // A transformed Pokémon keeps its copied stats.
+        if (!side.has(SUB5, C.SUBSTATUS_TRANSFORMED)) side.stats = mon.stats.slice(1);
+        // ApplyStatLevelMultiplierOnAllStats, ApplyStatusEffectOnPlayerStats,
+        // then BadgeStatBoosts.
+        this.applyStatLevelMultipliers(side);
+        this.applyStatusEffects(side);
+        this.badgeStatBoosts();
       }
       // LevelUpHappinessMod: more where it was caught.
       this.changeHappiness(