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

tests/NATS.Server.Tests/ServerTests.cs

@@ -1,6 +1,7 @@
 using System.Net;
 using System.Net.Sockets;
 using System.Text;
+using Microsoft.Extensions.Logging.Abstractions;
 using NATS.Server;
 
 namespace NATS.Server.Tests;
@@ -15,7 +16,7 @@ public class ServerTests : IAsyncDisposable
     {
         // Use random port
         _port = GetFreePort();
-        _server = new NatsServer(new NatsOptions { Port = _port });
+        _server = new NatsServer(new NatsOptions { Port = _port }, NullLoggerFactory.Instance);
     }
 
     public async ValueTask DisposeAsync()
@@ -54,7 +55,7 @@ public class ServerTests : IAsyncDisposable
         using var client = await ConnectClientAsync();
         var response = await ReadLineAsync(client);
 
-        Assert.StartsWith("INFO ", response);
+        response.ShouldStartWith("INFO ");
         await _cts.CancelAsync();
     }
 
@@ -84,7 +85,7 @@ public class ServerTests : IAsyncDisposable
         var n = await sub.ReceiveAsync(buf, SocketFlags.None);
         var msg = Encoding.ASCII.GetString(buf, 0, n);
 
-        Assert.Contains("MSG foo 1 5\r\nHello\r\n", msg);
+        msg.ShouldContain("MSG foo 1 5\r\nHello\r\n");
         await _cts.CancelAsync();
     }
 
@@ -110,7 +111,7 @@ public class ServerTests : IAsyncDisposable
         var n = await sub.ReceiveAsync(buf, SocketFlags.None);
         var msg = Encoding.ASCII.GetString(buf, 0, n);
 
-        Assert.Contains("MSG foo.bar 1 5\r\n", msg);
+        msg.ShouldContain("MSG foo.bar 1 5\r\n");
         await _cts.CancelAsync();
     }
 }