refactor(compress): move toFlushNow policy to Rust

Move the worker-count branch of ZSTD_toFlushNow into the Rust policy layer.
The C adapter retains the private CCtx and multithread context, exposing only
the worker count and a callback for the MT flush query.  Single-threaded
contexts keep the existing zero result and the ABI projection is checked on
both sides.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml to_flush_now --lib
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- ulimit -v 41943040; make -j1

lib/compress/zstd_compress.c

@@ -4020,18 +4020,40 @@ ZSTD_frameProgression ZSTD_getFrameProgression(const ZSTD_CCtx* cctx)
                                           cctx->producedCSize);
     }   }
 
+typedef size_t (*ZSTD_rust_toFlushNow_f)(void* context);
+typedef struct {
+    void* callbackContext;
+    int nbWorkers;
+    ZSTD_rust_toFlushNow_f toFlushNow;
+} ZSTD_rust_toFlushNowState;
+typedef char ZSTD_rust_to_flush_now_state_layout[
+        (offsetof(ZSTD_rust_toFlushNowState, callbackContext) == 0
+         && offsetof(ZSTD_rust_toFlushNowState, nbWorkers) == sizeof(void*)
+         && offsetof(ZSTD_rust_toFlushNowState, toFlushNow) == 2 * sizeof(void*)
+         && sizeof(ZSTD_rust_toFlushNowState) == 3 * sizeof(void*))
+        ? 1 : -1];
+size_t ZSTD_rust_toFlushNow(const ZSTD_rust_toFlushNowState* state);
+
+static size_t ZSTD_rust_toFlushNow_callback(void* context)
+{
+#ifdef ZSTD_MULTITHREAD
+    return ZSTDMT_toFlushNow(((ZSTD_CCtx*)context)->mtctx);
+#else
+    (void)context;
+    return 0;
+#endif
+}
+
 /*! ZSTD_toFlushNow()
  *  Only useful for multithreading scenarios currently (nbWorkers >= 1).
  */
 size_t ZSTD_toFlushNow(ZSTD_CCtx* cctx)
 {
-#ifdef ZSTD_MULTITHREAD
-    if (cctx->appliedParams.nbWorkers > 0) {
-        return ZSTDMT_toFlushNow(cctx->mtctx);
-    }
-#endif
-    (void)cctx;
-    return 0;   /* over-simplification; could also check if context is currently running in streaming mode, and in which case, report how many bytes are left to be flushed within output buffer */
+    ZSTD_rust_toFlushNowState state;
+    state.callbackContext = cctx;
+    state.nbWorkers = (int)cctx->appliedParams.nbWorkers;
+    state.toFlushNow = ZSTD_rust_toFlushNow_callback;
+    return ZSTD_rust_toFlushNow(&state);
 }
 
 static void ZSTD_assertEqualCParams(ZSTD_compressionParameters cParams1,