Don't use sleep incorrectly

design.md

@@ -298,6 +298,7 @@ This write-side component is designed to integrate with:
 - **Configuration**: All configuration is TOML-based using `config.toml` (see `config.md`)
 - **Testing Strategy**: Run unit tests, benchmarks, and debug tools before submitting changes
 - **Build System**: CMake generates gperf hash tables at build time; always use ninja
+- **Test Synchronization**: NEVER use sleep() for test synchronization - it makes tests slow and flaky. Use proper synchronization primitives like std::latch (C++20), condition variables, or promises/futures instead
 
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