feat(compress): move next input hint policy to Rust

Keep ZSTD_nextInputSizeHint's C-owned state-machine and MT routing while
moving its mode-dependent arithmetic behind a narrow scalar ABI. Rust now
receives the buffer mode and four size fields, preserving stable-buffer
capacity, buffered target remainder, and the zero-remainder block-size
fallback without exposing ZSTD_CCtx.

Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression next_input_size_hint` -- passed (3 tests)
- `make -B -C lib -j2 lib` -- passed
- `make -C tests -j2 fuzzer` and `./tests/fuzzer -s4560 -t47 -i48 -v` -- passed
- Required clippy, nightly fmt, and staged diff checks -- passed

lib/compress/zstd_compress.c

@@ -79,6 +79,11 @@ int ZSTD_rust_targetCBlockSizeAction(int bss, int isFirstBlock,
                                      int maybeRLE, int isRLE,
                                      size_t cSize, size_t srcSize,
                                      int strategy);
+size_t ZSTD_rust_nextInputSizeHint(int inBufferMode,
+                                   size_t blockSizeMax,
+                                   size_t stableInNotConsumed,
+                                   size_t inBuffTarget,
+                                   size_t inBuffPos);
 size_t ZSTD_rust_CStreamInSize(void);
 size_t ZSTD_rust_CStreamOutSize(void);
 
@@ -4524,14 +4529,20 @@ size_t ZSTD_initCStream(ZSTD_CStream* zcs, int compressionLevel)
 
 static size_t ZSTD_nextInputSizeHint(const ZSTD_CCtx* cctx)
 {
-    if (cctx->appliedParams.inBufferMode == ZSTD_bm_stable) {
-        return cctx->blockSizeMax - cctx->stableIn_notConsumed;
-    }
-    assert(cctx->appliedParams.inBufferMode == ZSTD_bm_buffered);
-    {   size_t hintInSize = cctx->inBuffTarget - cctx->inBuffPos;
-        if (hintInSize==0) hintInSize = cctx->blockSizeMax;
-        return hintInSize;
+    int const inBufferMode = (int)cctx->appliedParams.inBufferMode;
+    if (inBufferMode == ZSTD_bm_stable) {
+        return ZSTD_rust_nextInputSizeHint(inBufferMode,
+                                           cctx->blockSizeMax,
+                                           cctx->stableIn_notConsumed,
+                                           cctx->inBuffTarget,
+                                           cctx->inBuffPos);
     }
+    assert(inBufferMode == ZSTD_bm_buffered);
+    return ZSTD_rust_nextInputSizeHint(inBufferMode,
+                                       cctx->blockSizeMax,
+                                       cctx->stableIn_notConsumed,
+                                       cctx->inBuffTarget,
+                                       cctx->inBuffPos);
 }
 
 /** ZSTD_compressStream_generic():