feat: add structured logging, Shouldly assertions, CPM, and project documentation

- Add Microsoft.Extensions.Logging + Serilog to NatsServer and NatsClient
- Convert all test assertions from xUnit Assert to Shouldly
- Add NSubstitute package for future mocking needs
- Introduce Central Package Management via Directory.Packages.props
- Add documentation_rules.md with style guide, generation/update rules, component map
- Generate 10 documentation files across 5 component folders (GettingStarted, Protocol, Subscriptions, Server, Configuration/Operations)
- Update CLAUDE.md with logging, testing, porting, agent model, CPM, and documentation guidance

src/NATS.Server.Host/Program.cs

@@ -1,5 +1,11 @@
-// src/NATS.Server.Host/Program.cs
 using NATS.Server;
+using Serilog;
+
+Log.Logger = new LoggerConfiguration()
+    .MinimumLevel.Debug()
+    .Enrich.FromLogContext()
+    .WriteTo.Console(outputTemplate: "[{Timestamp:HH:mm:ss} {Level:u3}] {Message:lj}{NewLine}{Exception}")
+    .CreateLogger();
 
 var options = new NatsOptions();
 
@@ -20,7 +26,8 @@ for (int i = 0; i < args.Length; i++)
     }
 }
 
-var server = new NatsServer(options);
+using var loggerFactory = new Serilog.Extensions.Logging.SerilogLoggerFactory(Log.Logger);
+var server = new NatsServer(options, loggerFactory);
 
 var cts = new CancellationTokenSource();
 Console.CancelKeyPress += (_, e) =>
@@ -29,12 +36,12 @@ Console.CancelKeyPress += (_, e) =>
     cts.Cancel();
 };
 
-Console.WriteLine($"[NATS] Listening on {options.Host}:{options.Port}");
-
 try
 {
     await server.StartAsync(cts.Token);
 }
 catch (OperationCanceledException) { }
-
-Console.WriteLine("[NATS] Server stopped.");
+finally
+{
+    Log.CloseAndFlush();
+}