fix(battle-node): harden SIO parse + narrow Matched OppoId/Seed to int

#3: SocketIoFrame.Parse now range-checks the packet type char (was
unchecked cast — any char outside 0-6 produced an undefined enum
value) and uses int.TryParse for ack-id (was int.Parse — a >10-digit
ack-id threw OverflowException, tearing down the WS mid-game). Both
now throw ArgumentException consistently. The read loop in
RealParticipant wraps both EIO and SIO parse calls with try-catch so
a malformed frame is logged and skipped instead of killing the battle.

#4: MatchedSelfInfo/MatchedOppoInfo OppoId and Seed narrowed from
long to int. The client reads both with Convert.ToInt32 inside a
swallowing try/catch — any value > int.MaxValue silently dropped the
Matched event, preventing the battle from starting. Seed was already
int-range (BattleSeeds.Stable returns int); OppoId (viewer ID) is
~847M in captures, well under int.MaxValue. The narrowing cast now
happens explicitly in ServerBattleFrames.BuildMatched at the wire
boundary.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

SVSim.BattleNode/Lifecycle/ServerBattleFrames.cs

@@ -23,7 +23,7 @@ public static class ServerBattleFrames
     public static MsgEnvelope BuildMatched(
         MatchContext selfCtx, MatchContext oppoCtx,
         long selfViewerId, long oppoViewerId,
-        string battleId, long seed, IReadOnlyList<long> selfDeckOrder) =>
+        string battleId, int seed, IReadOnlyList<long> selfDeckOrder) =>
         EnvelopeForPush(NetworkBattleUri.Matched,
             new MatchedBody(
                 SelfInfo: new MatchedSelfInfo(
@@ -34,7 +34,7 @@ public static class ServerBattleFrames
                     DegreeId: selfCtx.DegreeId,
                     FieldId: selfCtx.FieldId,
                     IsOfficial: selfCtx.IsOfficial != 0,
-                    OppoId: oppoViewerId,
+                    OppoId: (int)oppoViewerId,
                     Seed: seed),
                 OppoInfo: new MatchedOppoInfo(
                     CountryCode: oppoCtx.CountryCode,
@@ -44,7 +44,7 @@ public static class ServerBattleFrames
                     DegreeId: oppoCtx.DegreeId,
                     FieldId: oppoCtx.FieldId,
                     IsOfficial: oppoCtx.IsOfficial != 0,
-                    OppoId: selfViewerId,
+                    OppoId: (int)selfViewerId,
                     Seed: seed,
                     OppoDeckCount: oppoCtx.SelfDeckCardIds.Count),
                 SelfDeck: BuildPlayerDeck(selfDeckOrder)),