config(battle-node): require NodeServerUrl from appsettings, no fallback

Move the localhost default off BattleNodeOptions and out of Program.cs
into an appsettings.json "BattleNode" section, and make AddBattleNode
throw at startup if the value is missing or blank. Deployments now must
set the URL explicitly, and dev keeps working via the checked-in value.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Program.cs

@@ -165,11 +165,10 @@ public class Program
 
         builder.Services.AddBattleNode(opt =>
         {
-            // Matches the prod do_matching wire format: host:port/socket.io/, no scheme prefix.
-            // BestHTTP's SocketManager parses this as the Socket.IO v2 endpoint URL.
-            opt.NodeServerUrl = "localhost:5148/socket.io/";
-            // Any field in BattleNodeOptions can be overridden via the "BattleNode" section
-            // in appsettings*.json — see appsettings.Development.json for DiagnosticLogging.
+            // Every field on BattleNodeOptions is populated from the "BattleNode" section in
+            // appsettings*.json. NodeServerUrl has no hardcoded fallback — AddBattleNode
+            // throws at startup if it's missing/blank. See BattleNodeOptions.NodeServerUrl
+            // for the required wire format.
             builder.Configuration.GetSection("BattleNode").Bind(opt);
         });
         // In-process FCFS pair-up for TK2 PvP /do_matching, plus rank-battle's AI-fallback