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/Wire/SocketIoFrame.cs

@@ -52,7 +52,10 @@ public sealed class SocketIoFrame
         if (string.IsNullOrEmpty(raw))
             throw new ArgumentException("Empty SIO payload", nameof(raw));
 
-        var type = (SocketIoPacketType)(raw[0] - '0');
+        var typeChar = raw[0];
+        if (typeChar < '0' || typeChar > '6')
+            throw new ArgumentException($"Invalid SIO type char '{typeChar}'", nameof(raw));
+        var type = (SocketIoPacketType)(typeChar - '0');
         var cursor = 1;
 
         var attachmentCount = 0;
@@ -84,7 +87,9 @@ public sealed class SocketIoFrame
         {
             var start = cursor;
             while (cursor < raw.Length && char.IsDigit(raw[cursor])) cursor++;
-            ackId = int.Parse(raw.AsSpan(start, cursor - start));
+            if (!int.TryParse(raw.AsSpan(start, cursor - start), out var parsedAckId))
+                throw new ArgumentException("SIO ack-id overflows int32", nameof(raw));
+            ackId = parsedAckId;
         }
 
         var argsJson = cursor < raw.Length ? raw.Substring(cursor) : string.Empty;