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

rust/src/zstd_compress.rs

@@ -72,6 +72,9 @@ unsafe extern "C" {
 
 const ZSTD_FAST: c_int = 1;
 const ZSTD_DFAST: c_int = 2;
+#[cfg(test)]
+const ZSTD_BM_BUFFERED: c_int = 0;
+const ZSTD_BM_STABLE: c_int = 1;
 const ZSTD_BLOCKSIZE_MAX: usize = 1 << 17;
 const ZSTD_CONTENTSIZE_UNKNOWN: u64 = u64::MAX;
 const ZSTD_TARGET_CBLOCK_BSS_COMPRESS: c_int = 0;
@@ -216,6 +219,44 @@ pub unsafe extern "C" fn ZSTD_rust_updateFrameProgression(
     ) as c_int
 }
 
+#[inline]
+fn next_input_size_hint(
+    in_buffer_mode: c_int,
+    block_size_max: usize,
+    stable_in_not_consumed: usize,
+    in_buff_target: usize,
+    in_buff_pos: usize,
+) -> usize {
+    if in_buffer_mode == ZSTD_BM_STABLE {
+        return block_size_max.wrapping_sub(stable_in_not_consumed);
+    }
+
+    let hint_in_size = in_buff_target.wrapping_sub(in_buff_pos);
+    if hint_in_size == 0 {
+        block_size_max
+    } else {
+        hint_in_size
+    }
+}
+
+/// Return the next input size required by the C streaming state machine.
+#[no_mangle]
+pub extern "C" fn ZSTD_rust_nextInputSizeHint(
+    in_buffer_mode: c_int,
+    block_size_max: usize,
+    stable_in_not_consumed: usize,
+    in_buff_target: usize,
+    in_buff_pos: usize,
+) -> usize {
+    next_input_size_hint(
+        in_buffer_mode,
+        block_size_max,
+        stable_in_not_consumed,
+        in_buff_target,
+        in_buff_pos,
+    )
+}
+
 #[inline]
 fn bitmix(mut val: u64, len: u64) -> u64 {
     val ^= val.rotate_right(49) ^ val.rotate_right(24);
@@ -999,6 +1040,30 @@ mod tests {
         assert_eq!(produced, 220);
     }
 
+    #[test]
+    fn next_input_size_hint_uses_remaining_stable_block_capacity() {
+        let hint = ZSTD_rust_nextInputSizeHint(ZSTD_BM_STABLE, 256, 37, 99, 12);
+
+        assert_eq!(next_input_size_hint(ZSTD_BM_STABLE, 256, 37, 99, 12), 219);
+        assert_eq!(hint, 219);
+    }
+
+    #[test]
+    fn next_input_size_hint_replaces_empty_buffered_hint_with_block_size() {
+        assert_eq!(
+            ZSTD_rust_nextInputSizeHint(ZSTD_BM_BUFFERED, 256, 37, 128, 128),
+            256
+        );
+    }
+
+    #[test]
+    fn next_input_size_hint_returns_nonzero_buffered_hint() {
+        assert_eq!(
+            ZSTD_rust_nextInputSizeHint(ZSTD_BM_BUFFERED, 256, 37, 128, 32),
+            96
+        );
+    }
+
     #[test]
     fn public_one_shot_abi_is_c_compatible() {
         let entry: unsafe extern "C" fn(*mut c_void, usize, *const c_void, usize, c_int) -> usize =