Rust frontend RTL — thread / atomics / mpsc shims

• Type: feature — Track B
• Status: backlog
• Owner: —
• Opened: 2026-07-03
• Umbrella: [[feature-rust-frontend]] — sub-ticket 11/12. Depends on [[feature-rust-frontend-skeleton]] and [[feature-rust-generics-trait-bounds]] (JoinHandle<T> is generic).

What it does

Pure API-shape wrappers — every underlying primitive already exists in lib/rtl, confirmed by reading the tree, nothing new at the runtime level:

• std::thread::spawn/JoinHandle<T> → lib/rtl/palthreadobj.pas (TThread-style object already handles create/join).
• std::sync::mpsc::channel → lib/rtl/channel.pas (already exists).
• AtomicBool/AtomicU64/etc. → __pxxatomic_xchg/cas/add intrinsics (already exist, AN_ATOMIC defs.inc ~263).
• Arc<T> around these → existing ARC machinery, same as [[feature-rust-rtl-core-types]]'s Box/Rc reuse.

Real-world confirmation: ~/nextlevel/engine/src/search.rs + src/uci.rs use std::thread::spawn, JoinHandle, mpsc::channel, AtomicBool, AtomicU64, Arc<AtomicBool> — this is the concurrency surface the target app actually needs, nothing more exotic (no RwLock/Condvar in that codebase as of the audit).

Acceptance

• thread::spawn(move || ...) + .join() round-trips a closure through palthreadobj.pas's existing thread object, return value flows through JoinHandle<T>::join() -> T correctly (generic — needs [[feature-rust-generics-trait-bounds]]).
• mpsc::channel() send/recv across threads works via channel.pas.
• Atomic load/store/CAS on AtomicBool/AtomicU64 match the requested Ordering semantics closely enough for correct producer/consumer code (x86-64: most orderings collapse to the same instruction anyway).

Log

• 2026-07-03 — split from [[feature-rust-frontend]] umbrella at ticket-craft time. No code written yet.