Port Reflect and Light Screen

BattleCommand_Screen and HandleScreens run natively. Each side owns
independent five-turn counters that include the setup turn, do not
refresh on repeated use and survive enemy send-outs. Screens expire in
the original non-Link order with ROM messages; the existing damage path
already doubles the right defense and keeps the critical/stage rule.
This closes the only refusal found by the Route 30 stress run
(Chikorita's Reflect).

The NPC movement test now reads the ROM in beforeAll, so it skips
cleanly without one.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/game/battle/battle.ts

@@ -61,6 +61,8 @@ class Side {
   wrapCount = 0;
   damageTaken = 0;
   screens = 0;
+  lightScreenCount = 0;
+  reflectCount = 0;
   turnsTaken = 0;
   charging = 0;
   lastMove = 0;
@@ -1208,6 +1210,8 @@ export class Battle {
         return this.user.set(SUB4, C.SUBSTATUS_RAGE);
       case COMMAND.ragedamage:
         return this.rageDamage();
+      case COMMAND.screen:
+        return yield* this.screen();
       case COMMAND.curl:
         return this.user.set(SUB2, C.SUBSTATUS_CURLED);
       case COMMAND.doubleflyingdamage:
@@ -1302,6 +1306,41 @@ export class Battle {
     if (this.random() < this.data.criticalChance(stage)) this.criticalHit = 1;
   }
 
+  /** BattleCommand_Screen: a repeated screen fails without renewing it. */
+  private *screen(): Gen {
+    const side = this.user;
+    const light = side.move.effect === C.EFFECT_LIGHT_SCREEN;
+    const bit = 1 << (light ? C.SCREENS_LIGHT_SCREEN : C.SCREENS_REFLECT);
+    if (side.screens & bit) return yield* this.say(BATTLE.ButItFailedText);
+    side.screens |= bit;
+    if (light) side.lightScreenCount = 5;
+    else side.reflectCount = 5;
+    yield* this.say(light ? BATTLE.LightScreenEffectText : BATTLE.ReflectEffectText);
+  }
+
+  /** HandleScreens, in non-Link order. Screens belong to the side and
+   * survive a send-out; their setup turn is the first of five ticks. */
+  private *handleScreens(): Gen {
+    for (const [turn, side] of [this.player, this.enemy].entries()) {
+      this.turn = turn;
+      this.strings[RAM.stringBuffer1] = turn === 0 ? "Your" : "Enemy";
+      if (side.screens & (1 << C.SCREENS_LIGHT_SCREEN)) {
+        side.lightScreenCount = (side.lightScreenCount - 1) & 0xff;
+        if (!side.lightScreenCount) {
+          side.screens &= ~(1 << C.SCREENS_LIGHT_SCREEN);
+          yield* this.say(BATTLE.BattleText_MonsLightScreenFell);
+        }
+      }
+      if (side.screens & (1 << C.SCREENS_REFLECT)) {
+        side.reflectCount = (side.reflectCount - 1) & 0xff;
+        if (!side.reflectCount) {
+          side.screens &= ~(1 << C.SCREENS_REFLECT);
+          yield* this.say(BATTLE.BattleText_MonsReflectFaded);
+        }
+      }
+    }
+  }
+
   private damageArgs = { power: 0, level: 0, attack: 0, defense: 0 };
   /** PlayerAttackDamage / EnemyAttackDamage. */
   private damageStats() {
@@ -1968,7 +2007,8 @@ export class Battle {
     for (const side of [this.player, this.enemy]) {
       if (side.wrapCount || side.has(SUB1, C.SUBSTATUS_PERISH) || side.has(SUB5, C.SUBSTATUS_ENCORED))
         throw new UnsupportedBattle("Wrap, Perish Song and Encore are not ported yet.");
-      if (side.screens) throw new UnsupportedBattle("Screens and Safeguard are not ported yet.");
+      if (side.screens & ~((1 << C.SCREENS_LIGHT_SCREEN) | (1 << C.SCREENS_REFLECT)))
+        throw new UnsupportedBattle("Other screen effects are not ported yet.");
       const effect = this.data.heldEffect(side.mon.item).effect;
       if (effect === C.HELD_RESTORE_PP || (effect >= C.HELD_ATTACK_UP && effect <= C.HELD_EVASION_UP))
         throw new UnsupportedBattle("Mystery Berry and stat-raising held items are not ported yet.");
@@ -1995,6 +2035,7 @@ export class Battle {
       yield this.hud();
       yield* this.say(BATTLE.DefrostedOpponentText);
     }
+    yield* this.handleScreens();
     // HandleHealingItems: turn 0 handles the enemy's items, then turn 1.
     for (const turn of [0, 1]) {
       this.turn = turn;