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/NatsClient.cs

@@ -3,6 +3,7 @@ using System.IO.Pipelines;
 using System.Net.Sockets;
 using System.Text;
 using System.Text.Json;
+using Microsoft.Extensions.Logging;
 using NATS.Server.Protocol;
 using NATS.Server.Subscriptions;
 
@@ -29,6 +30,7 @@ public sealed class NatsClient : IDisposable
     private readonly NatsParser _parser;
     private readonly SemaphoreSlim _writeLock = new(1, 1);
     private readonly Dictionary<string, Subscription> _subs = new();
+    private readonly ILogger _logger;
 
     public ulong Id { get; }
     public ClientOptions? ClientOpts { get; private set; }
@@ -43,13 +45,14 @@ public sealed class NatsClient : IDisposable
 
     public IReadOnlyDictionary<string, Subscription> Subscriptions => _subs;
 
-    public NatsClient(ulong id, Socket socket, NatsOptions options, ServerInfo serverInfo)
+    public NatsClient(ulong id, Socket socket, NatsOptions options, ServerInfo serverInfo, ILogger logger)
     {
         Id = id;
         _socket = socket;
         _stream = new NetworkStream(socket, ownsSocket: false);
         _options = options;
         _serverInfo = serverInfo;
+        _logger = logger;
         _parser = new NatsParser(options.MaxPayload);
     }
 
@@ -68,7 +71,10 @@ public sealed class NatsClient : IDisposable
             await Task.WhenAny(fillTask, processTask);
         }
         catch (OperationCanceledException) { }
-        catch (Exception) { /* connection error -- clean up */ }
+        catch (Exception ex)
+        {
+            _logger.LogDebug(ex, "Client {ClientId} connection error", Id);
+        }
         finally
         {
             Router?.RemoveClient(this);
@@ -160,6 +166,7 @@ public sealed class NatsClient : IDisposable
         ClientOpts = JsonSerializer.Deserialize<ClientOptions>(cmd.Payload.Span)
                      ?? new ClientOptions();
         ConnectReceived = true;
+        _logger.LogDebug("CONNECT received from client {ClientId}, name={ClientName}", Id, ClientOpts?.Name);
     }
 
     private void ProcessSub(ParsedCommand cmd)
@@ -174,12 +181,16 @@ public sealed class NatsClient : IDisposable
         _subs[cmd.Sid!] = sub;
         sub.Client = this;
 
+        _logger.LogDebug("SUB {Subject} {Sid} from client {ClientId}", cmd.Subject, cmd.Sid, Id);
+
         if (Router is ISubListAccess sl)
             sl.SubList.Insert(sub);
     }
 
     private void ProcessUnsub(ParsedCommand cmd)
     {
+        _logger.LogDebug("UNSUB {Sid} from client {ClientId}", cmd.Sid, Id);
+
         if (!_subs.TryGetValue(cmd.Sid!, out var sub))
             return;