fix(compress): keep MT frame callback warning-clean

The Rust-owned frame-progression dispatch invokes the existing C MT progression API through a mutable opaque callback context. Match that API's established signature in the adapter so the new boundary does not introduce a discarded-const qualifier warning while preserving the private MT context.

Test Plan:

- git diff --cached --check

- Capped native make rerun pending after this warning-only fix

lib/compress/zstd_compress.c

@@ -4442,7 +4442,7 @@ size_t ZSTD_estimateCStreamSize(int compressionLevel)
 static ZSTD_frameProgression ZSTD_rust_frameProgression_MT(void* context)
 {
 #ifdef ZSTD_MULTITHREAD
-    return ZSTDMT_getFrameProgression((const ZSTDMT_CCtx*)context);
+    return ZSTDMT_getFrameProgression((ZSTDMT_CCtx*)context);
 #else
     (void)context;
     return (ZSTD_frameProgression){ 0 };