Send + Sync marker traits

Rust's Send and Sync auto-marker traits let the compiler prove at build time whether a value can be moved across threads or shared by reference, turning data races into compile errors.

grounded in: latest AI/tech trends theme 'Rust concurrency craft: Deep systems debate around async runtimes and Rust-style design', grounded in this system's Rust codebase (existing concepts rust, borrow-checker,