Record Atomicpub

Atomic cell over a word-sized primitive type. All ops take an explicit MemoryOrdering. The WP-F1 followup wires the real host intrinsics on top of Rust's std::sync::atomic::AtomicI64 — see src/frontend/intrinsics/sync.rs. Two caller-visible simplifications documented there:

1. Every Atomic t is backed by an AtomicI64 at runtime, regardless of t. Fine for any Silo integer scalar.
2. Every op uses SeqCst ordering regardless of the MemoryOrdering argument the caller supplies. The argument is still required so the checker-visible signature matches the spec; it is consumed-and-ignored by the host.