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
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