refactor(battle-node): unify IMatchingBridge.RegisterBattle signature

Single RegisterBattle(p1, p2?, type) with contract validation throws on
invalid combinations (Pvp requires both; Bot requires p2==null; Scripted
accepts either). PendingBattle carries Type + P1 + nullable P2. Handler
+ controller adapt; v1.2 behaviour preserved because Scripted is the
only type used today (Phase 2 adds Pvp, Phase 3 adds Bot).

SVSim.EmulatedEntrypoint/Controllers/ArenaTwoPickBattleController.cs

@@ -30,7 +30,10 @@ public class ArenaTwoPickBattleController : SVSimController
         try
         {
             var ctx = await _matchContextBuilder.BuildForTwoPickAsync(vid);
-            var match = _matching.RegisterPendingBattle(vid, ctx);
+            var match = _matching.RegisterBattle(
+                new SVSim.BattleNode.Bridge.BattlePlayer(vid, ctx),
+                p2: null,
+                SVSim.BattleNode.Sessions.BattleType.Scripted);
             return Ok(new DoMatchingResponseDto
             {
                 MatchingState = 3004,