feat(compress): move thread-pool attachment policy into Rust

Move ZSTD_CCtx_refThreadPool init-stage validation and pool-slot assignment
behind a Rust-owned ABI bridge while keeping the opaque pool and CCtx storage
in C. Preserve the stage error and avoid mutating the pool on rejection.

Test Plan:
- cargo test --manifest-path rust/Cargo.toml --lib
- cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- make -B -C programs -j1 zstd
- make -C tests -j1 test-zstream ZSTREAM_TESTTIME=-T1s
- focused ref_thread_pool unit tests

lib/compress/zstd_compress.c

@@ -287,6 +287,20 @@ typedef char ZSTD_rust_set_parameter_state_layout[
                 == 4 * sizeof(void*)
          && sizeof(ZSTD_rust_setParameterState) == 5 * sizeof(void*))
         ? 1 : -1];
+/* Thread-pool attachment policy lives in Rust.  The pool slot remains
+ * C-owned; this projection passes its storage location and opaque value. */
+typedef struct {
+    void** pool;
+    void* requestedPool;
+    int streamStage;
+} ZSTD_rust_refThreadPoolState;
+size_t ZSTD_rust_refThreadPool(const ZSTD_rust_refThreadPoolState* state);
+typedef char ZSTD_rust_ref_thread_pool_state_layout[
+        (offsetof(ZSTD_rust_refThreadPoolState, pool) == 0
+         && offsetof(ZSTD_rust_refThreadPoolState, requestedPool) == sizeof(void*)
+         && offsetof(ZSTD_rust_refThreadPoolState, streamStage) == 2 * sizeof(void*)
+         && sizeof(ZSTD_rust_refThreadPoolState) == 3 * sizeof(void*))
+        ? 1 : -1];
 typedef void (*ZSTD_rust_resetCCtxClearAllDicts_f)(void* context);
 typedef size_t (*ZSTD_rust_resetCCtxResetParams_f)(void* context);
 typedef struct {
@@ -2084,10 +2098,11 @@ size_t ZSTD_CCtx_refCDict(ZSTD_CCtx* cctx, const ZSTD_CDict* cdict)
 
 size_t ZSTD_CCtx_refThreadPool(ZSTD_CCtx* cctx, ZSTD_threadPool* pool)
 {
-    RETURN_ERROR_IF(cctx->streamStage != zcss_init, stage_wrong,
-                    "Can't ref a pool when ctx not in init stage.");
-    cctx->pool = pool;
-    return 0;
+    ZSTD_rust_refThreadPoolState state;
+    state.pool = (void**)&cctx->pool;
+    state.requestedPool = pool;
+    state.streamStage = (int)cctx->streamStage;
+    return ZSTD_rust_refThreadPool(&state);
 }
 
 size_t ZSTD_CCtx_refPrefix(ZSTD_CCtx* cctx, const void* prefix, size_t prefixSize)