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/appsettings.json

@@ -9,6 +9,18 @@
       "Microsoft.EntityFrameworkCore.Infrastructure": "Warning"
     }
   },
+  "Serilog": {
+    "MinimumLevel": {
+      "Default": "Information",
+      "Override": {
+        "Microsoft.AspNetCore": "Warning",
+        "Microsoft.AspNetCore.HttpLogging.HttpLoggingMiddleware": "Information",
+        "Microsoft.EntityFrameworkCore": "Warning",
+        "Microsoft.EntityFrameworkCore.Database.Command": "Warning",
+        "Microsoft.EntityFrameworkCore.Infrastructure": "Warning"
+      }
+    }
+  },
   "ConnectionStrings": {
     "ApplicationDb": "Host=localhost;Database=svsim;Username=postgres;password=postgres"
   },