feat(rust): port worker pool and pthread wrappers

Replace the common worker pool and debug pthread wrappers with ABI-compatible
Rust implementations. The pool preserves bounded-queue, resize, drain-on-free,
custom-allocator, and no-thread behavior while C supplies only the build-time
multithreading configuration bit.

This moves runtime support without changing C tests or public headers. Codec
and CLI implementations remain C while their dependencies are migrated.

Test Plan:
- cargo fmt --check
- cargo test --all-targets
- cargo clippy --all-targets -- -D warnings
- cargo build --release
- make -C tests poolTests
- ./tests/poolTests
- compile pool and threading shims with -Werror in MT and no-thread modes

Refs: rust/README.md

rust/src/lib.rs

@@ -9,5 +9,7 @@ pub mod entropy_common;
 pub mod errors;
 pub mod fse_decompress;
 pub mod mem;
+pub mod pool;
+pub mod threading;
 pub mod xxhash;
 pub mod zstd_common;