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/Bridge/IMatchingBridge.cs

@@ -1,12 +1,25 @@
+using SVSim.BattleNode.Sessions;
+
 namespace SVSim.BattleNode.Bridge;
 
 public interface IMatchingBridge
 {
     /// <summary>
-    /// Mint a battle id, register a pending session for the given viewer with their per-battle
-    /// MatchContext snapshot, and return the URL the client should open a socket to.
+    /// Mint a battle id, register a pending session, return the URL the client should
+    /// open a socket to.
     /// </summary>
-    PendingMatch RegisterPendingBattle(long viewerId, MatchContext context);
+    /// <remarks>
+    /// Contract rules (enforced; throws <see cref="ArgumentException"/>):
+    /// <list type="bullet">
+    ///   <item><c>Pvp</c>: <paramref name="p2"/> required. Both viewers expected to
+    ///         connect WS within 60s.</item>
+    ///   <item><c>Bot</c>: <paramref name="p2"/> must be null. One viewer expected;
+    ///         opponent runs in client.</item>
+    ///   <item><c>Scripted</c>: <paramref name="p2"/> currently null; future
+    ///         server-driven bot config rides on <paramref name="p2"/>.</item>
+    /// </list>
+    /// </remarks>
+    PendingMatch RegisterBattle(BattlePlayer p1, BattlePlayer? p2, BattleType type);
 }
 
 public sealed record PendingMatch(string BattleId, string NodeServerUrl);

SVSim.BattleNode/Bridge/MatchingBridge.cs

@@ -5,10 +5,8 @@ namespace SVSim.BattleNode.Bridge;
 
 /// <summary>
 /// In-process implementation of <see cref="IMatchingBridge"/>. The HTTP-side
-/// <c>do_matching</c> controller calls <see cref="RegisterPendingBattle"/>, which mints a
-/// 12-digit decimal battle id, stashes a <see cref="PendingBattle"/> entry in the
-/// <see cref="IBattleSessionStore"/>, and returns the node URL the client should connect to.
-/// The WebSocket handler resolves the same battle id back to the viewer on connect.
+/// matching queue calls <see cref="RegisterBattle"/> once it has decided "these two
+/// play each other" or "this viewer is solo (bot/scripted)."
 /// </summary>
 public sealed class MatchingBridge : IMatchingBridge
 {
@@ -21,17 +19,39 @@ public sealed class MatchingBridge : IMatchingBridge
         _options = options;
     }
 
-    public PendingMatch RegisterPendingBattle(long viewerId, MatchContext context)
+    public PendingMatch RegisterBattle(BattlePlayer p1, BattlePlayer? p2, BattleType type)
     {
+        ValidateContract(p1, p2, type);
+
         // 12-digit decimal battle id mirrors the captures (e.g. "975695075012").
         // Two unbiased 6-digit draws concatenated — RandomNumberGenerator.GetInt32 uses
-        // rejection sampling so the result is uniform on [0, 10^6). The previous
-        // implementation (Guid.GetHashCode + mod) collapsed 128 bits into ~32 of entropy
-        // and tripped Math.Abs(int.MinValue) UB on the unlucky hash.
+        // rejection sampling so the result is uniform on [0, 10^6).
         var hi = RandomNumberGenerator.GetInt32(0, 1_000_000);
         var lo = RandomNumberGenerator.GetInt32(0, 1_000_000);
         var battleId = $"{hi:D6}{lo:D6}";
-        _store.RegisterPending(new PendingBattle(battleId, viewerId, context));
+
+        _store.RegisterPending(new PendingBattle(battleId, type, p1, p2));
         return new PendingMatch(battleId, _options.NodeServerUrl);
     }
+
+    private static void ValidateContract(BattlePlayer p1, BattlePlayer? p2, BattleType type)
+    {
+        if (p1 is null) throw new ArgumentNullException(nameof(p1));
+        switch (type)
+        {
+            case BattleType.Pvp:
+                if (p2 is null) throw new ArgumentException("Pvp requires both p1 and p2.", nameof(p2));
+                if (p1.ViewerId == p2.ViewerId)
+                    throw new ArgumentException("Pvp requires distinct viewer ids.", nameof(p2));
+                break;
+            case BattleType.Bot:
+                if (p2 is not null) throw new ArgumentException("Bot must have p2==null.", nameof(p2));
+                break;
+            case BattleType.Scripted:
+                // p2 currently null; future server-driven bot will populate it.
+                break;
+            default:
+                throw new ArgumentOutOfRangeException(nameof(type), type, "Unknown BattleType.");
+        }
+    }
 }

SVSim.BattleNode/Hosting/BattleNodeWebSocketHandler.cs

@@ -93,18 +93,20 @@ public sealed class BattleNodeWebSocketHandler
             ctx.Response.StatusCode = StatusCodes.Status404NotFound;
             return;
         }
-        if (pending.ViewerId != viewerId)
+        if (pending.P1.ViewerId != viewerId)
         {
             _log.LogWarning(
                 "WS upgrade viewer-id mismatch on BattleId={Bid}: bridge expected={Expected}, decrypted={Got}.",
-                battleId, pending.ViewerId, viewerId);
+                battleId, pending.P1.ViewerId, viewerId);
             ctx.Response.StatusCode = StatusCodes.Status401Unauthorized;
             return;
         }
 
         var ws = await ctx.WebSockets.AcceptWebSocketAsync();
         _store.RemovePending(battleId);
-        var session = new BattleSession(ws, battleId, viewerId, pending.Context, _loggerFactory.CreateLogger<BattleSession>());
+        // Phase 1: handler still constructs the old single-WS BattleSession.
+        // Task 9 switches to BattleSessionV2 + RealParticipant + ScriptedBotParticipant.
+        var session = new BattleSession(ws, battleId, viewerId, pending.P1.Context, _loggerFactory.CreateLogger<BattleSession>());
         await session.RunAsync(ctx.RequestAborted);
     }
 

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);