refactor(battle-node): tighten Phase setter to private; document sid opacity

SVSim.BattleNode/Sessions/BattleSession.cs

@@ -33,7 +33,7 @@ public sealed class BattleSession
 
     public string BattleId { get; }
     public long ViewerId { get; }
-    public BattleSessionPhase Phase { get; internal set; } = BattleSessionPhase.AwaitingInitNetwork;
+    public BattleSessionPhase Phase { get; private set; } = BattleSessionPhase.AwaitingInitNetwork;
     public InboundTracker Inbound { get; } = new();
     public OutboundSequencer Outbound { get; } = new();
 
@@ -378,6 +378,10 @@ public sealed class BattleSession
 
     private async Task SendEioOpenAsync(CancellationToken ct)
     {
+        // SID format is server-internal; the EIO3 spec suggests a 20-char base64-url id,
+        // but BestHTTP's SocketIO client treats it as an opaque string and never validates
+        // shape. Hex-truncated Guid is fine in practice; revisit if we ever care about
+        // matching prod telemetry.
         var sid = Guid.NewGuid().ToString("N").Substring(0, 16);
         var handshake = new EngineIoHandshake(sid, Array.Empty<string>(), 25000, 60000).ToJson();
         await SendTextAsync($"0{handshake}", ct);