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/NatsServer.cs

@@ -1,6 +1,7 @@
 using System.Collections.Concurrent;
 using System.Net;
 using System.Net.Sockets;
+using Microsoft.Extensions.Logging;
 using NATS.Server.Protocol;
 using NATS.Server.Subscriptions;
 
@@ -12,14 +13,18 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     private readonly ConcurrentDictionary<ulong, NatsClient> _clients = new();
     private readonly SubList _subList = new();
     private readonly ServerInfo _serverInfo;
+    private readonly ILogger<NatsServer> _logger;
+    private readonly ILoggerFactory _loggerFactory;
     private Socket? _listener;
     private ulong _nextClientId;
 
     public SubList SubList => _subList;
 
-    public NatsServer(NatsOptions options)
+    public NatsServer(NatsOptions options, ILoggerFactory loggerFactory)
     {
         _options = options;
+        _loggerFactory = loggerFactory;
+        _logger = loggerFactory.CreateLogger<NatsServer>();
         _serverInfo = new ServerInfo
         {
             ServerId = Guid.NewGuid().ToString("N")[..20].ToUpperInvariant(),
@@ -40,6 +45,8 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
             _options.Port));
         _listener.Listen(128);
 
+        _logger.LogInformation("Listening on {Host}:{Port}", _options.Host, _options.Port);
+
         try
         {
             while (!ct.IsCancellationRequested)
@@ -47,7 +54,10 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
                 var socket = await _listener.AcceptAsync(ct);
                 var clientId = Interlocked.Increment(ref _nextClientId);
 
-                var client = new NatsClient(clientId, socket, _options, _serverInfo);
+                _logger.LogDebug("Client {ClientId} connected from {RemoteEndpoint}", clientId, socket.RemoteEndPoint);
+
+                var clientLogger = _loggerFactory.CreateLogger($"NATS.Server.NatsClient[{clientId}]");
+                var client = new NatsClient(clientId, socket, _options, _serverInfo, clientLogger);
                 client.Router = this;
                 _clients[clientId] = client;
 
@@ -69,6 +79,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
         }
         finally
         {
+            _logger.LogDebug("Client {ClientId} disconnected", client.Id);
             RemoveClient(client);
         }
     }
@@ -127,6 +138,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     public void RemoveClient(NatsClient client)
     {
         _clients.TryRemove(client.Id, out _);
+        _logger.LogDebug("Removed client {ClientId}", client.Id);
         client.RemoveAllSubscriptions(_subList);
     }