[R29] Update the README storage example to EntityStorage

[JetF0x]

Addresses review finding R29.

Copying the README's server storage example failed with CS0246 because it implemented the removed IEntityStorage interface. The replacement derives from EntityStorage, implements the protected enumeration/add/remove hooks and public lookup/relevancy methods, and preserves the nullable lookup contract. The README explains the base-owned registration, ID assignment and relay attachment operations, and separates the class declaration from the setup statements.

Add an external tests/ReadmeStorageSmoke consumer that compiles the complete documented storage implementation with all required runtime/analyzer/generator references. It checks that the compiled source matches the README and exercises registration, lookup, attachment before and after server construction, relevancy, relay replication, and removal through storage calls and client messages.

Validation on .NET SDK 10.0.400:

• dotnet run --project tests/ReadmeStorageSmoke/ReadmeStorageSmoke.csproj --configuration Release --no-restore passed targeting .NET 8 with C# 14 and nullable warnings treated as errors in the consumer.
• dotnet test Cat.Network.Test/Cat.Network.Test.csproj --configuration Release --no-restore --verbosity minimal passed all 155 existing runtime tests.
• git diff HEAD^ HEAD --check passed. The library's existing CS8600 warning at MemoryRelayTransport.cs:21 remains.

Compatibility: documentation and a standalone consumer fixture only; no production API, wire format, or dependency changes. The client smoke completes the existing handshake before spawning and is independent of the separate immediate-spawn fix.