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.BattleNode/Sessions/PendingBattle.cs

@@ -3,8 +3,8 @@ using SVSim.BattleNode.Bridge;
 namespace SVSim.BattleNode.Sessions;
 
 /// <summary>
-/// Sparse pre-connect record: viewer id + the per-battle MatchContext snapshot. Enough to
-/// validate the incoming WS connect, resolve the viewer, and seed the BattleSession with the
-/// player-half lifecycle data. Full BattleSession is created on connect.
+/// Sparse pre-connect record. Carries the battle type + one or two players. The
+/// WebSocket handler reads this to validate the incoming WS connect and to
+/// construct the right participants.
 /// </summary>
-public sealed record PendingBattle(string BattleId, long ViewerId, MatchContext Context);
+public sealed record PendingBattle(string BattleId, BattleType Type, BattlePlayer P1, BattlePlayer? P2);