app: wire Serilog into SVSim.EmulatedEntrypoint

Registers Serilog via SVSim.Hosting.UseSvSimSerilog("svsim-api") right
after CreateBuilder and wraps Main in try/catch/finally so a fatal
startup exception logs before CloseAndFlush.

Also updates the SVSim.Hosting extension to gate the file sink on entry
assembly rather than the ASP.NET environment name. WebApplicationFactory
sets the environment via a hook that resolves after both CreateBuilder
and the UseSerilog callback see their environment view, so
IsEnvironment("Testing") is unreliable in tests. The entry assembly
under `dotnet test` is always "testhost", which is a reliable signal.

appsettings.json grows a Serilog section mirroring the existing
Logging:LogLevel overrides; the Logging block itself is left in place
per the spec's follow-up cleanup note.

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

SVSim.EmulatedEntrypoint/Program.cs

@@ -20,6 +20,8 @@ using SVSim.EmulatedEntrypoint.Middlewares;
 using SVSim.EmulatedEntrypoint.Security.SteamSessionAuthentication;
 using SVSim.EmulatedEntrypoint.Services;
 using SVSim.BattleNode.Hosting;
+using Serilog;
+using SVSim.Hosting;
 
 namespace SVSim.EmulatedEntrypoint;
 
@@ -29,6 +31,13 @@ public class Program
     {
         var builder = WebApplication.CreateBuilder(args);
 
+        // Always register Serilog. The file sink self-gates on the Testing environment inside
+        // the UseSvSimSerilog callback — a caller-side check here would be too early because
+        // WebApplicationFactory applies UseEnvironment("Testing") after CreateBuilder returns.
+        builder.Host.UseSvSimSerilog("svsim-api");
+
+        try
+        {
         // Add services to the container.
 
         builder.Services.AddControllers().AddJsonOptions(opt =>
@@ -253,5 +262,15 @@ public class Program
         app.MapControllers();
 
         app.Run();
+        }
+        catch (Exception ex)
+        {
+            Log.Fatal(ex, "Host terminated unexpectedly");
+            throw;
+        }
+        finally
+        {
+            Log.CloseAndFlush();
+        }
     }
 }