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/SubjectMatchTests.cs

@@ -23,7 +23,7 @@ public class SubjectMatchTests
     [InlineData("foo\tbar", false)]   // no tabs
     public void IsValidSubject(string subject, bool expected)
     {
-        Assert.Equal(expected, SubjectMatch.IsValidSubject(subject));
+        SubjectMatch.IsValidSubject(subject).ShouldBe(expected);
     }
 
     [Theory]
@@ -35,7 +35,7 @@ public class SubjectMatchTests
     [InlineData("*", false)]
     public void IsValidPublishSubject(string subject, bool expected)
     {
-        Assert.Equal(expected, SubjectMatch.IsValidPublishSubject(subject));
+        SubjectMatch.IsValidPublishSubject(subject).ShouldBe(expected);
     }
 
     [Theory]
@@ -52,6 +52,6 @@ public class SubjectMatchTests
     [InlineData("foo.bar", "foo.bar.>", false)]
     public void MatchLiteral(string literal, string pattern, bool expected)
     {
-        Assert.Equal(expected, SubjectMatch.MatchLiteral(literal, pattern));
+        SubjectMatch.MatchLiteral(literal, pattern).ShouldBe(expected);
     }
 }