feat(compress): move end-flush input policy to Rust

Keep ZSTD_CStream context access and caller-specific streaming behavior in C,
while moving the pure stable-versus-buffered input-buffer policy to a scalar
Rust ABI helper. The helper returns the public repr(C) buffer layout, preserves
stable-mode source, size, and position values including NULL sources, and
returns a zeroed buffer for every other mode.

Test Plan:
- `cargo clippy --manifest-path rust/Cargo.toml --no-default-features --features compression` -- passed before and after formatting
- the same clippy command with `--benches` and `--tests` -- passed before and after formatting
- `cargo +nightly fmt --manifest-path rust/Cargo.toml` -- passed
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression` -- 274 passed
- `make -B -C lib -j2 lib` -- passed
- `make -C tests test-rust-lib-smoke` -- passed
- `make -B -C tests -j2 fuzzer` and `tests/fuzzer -s4560 -t56 -i57 -v` -- passed
- `make -C tests -j2 test-zstream` -- 84 named tests and 7,400 plus 8,519 randomized cases passed
- `git diff --check` and `git diff --cached --check` -- passed

rust/src/zstd_compress.rs

@@ -159,7 +159,6 @@ pub extern "C" fn ZSTD_rust_targetCBlockSizeAction(
     ) as c_int
 }
 
-#[cfg(not(test))]
 #[repr(C)]
 pub struct ZSTD_inBuffer {
     src: *const c_void,
@@ -175,6 +174,28 @@ pub struct ZSTD_outBuffer {
     pos: usize,
 }
 
+#[no_mangle]
+pub extern "C" fn ZSTD_rust_inBufferForEndFlush(
+    in_buffer_mode: c_int,
+    expected_src: *const c_void,
+    expected_size: usize,
+    expected_pos: usize,
+) -> ZSTD_inBuffer {
+    if in_buffer_mode == ZSTD_BM_STABLE {
+        ZSTD_inBuffer {
+            src: expected_src,
+            size: expected_size,
+            pos: expected_pos,
+        }
+    } else {
+        ZSTD_inBuffer {
+            src: ptr::null(),
+            size: 0,
+            pos: 0,
+        }
+    }
+}
+
 /* HUF_WORKSPACE_SIZE + (MaxSeq + 2) * sizeof(unsigned), rounded up.  The
  * superblock leaf also accepts the larger pre-split workspace, so a fixed
  * 16 KiB buffer is sufficient for this first non-splitting path on both
@@ -1147,6 +1168,35 @@ mod tests {
         );
     }
 
+    #[test]
+    fn in_buffer_for_end_flush_returns_stable_expected_buffer() {
+        let expected_src = b"input".as_ptr().cast::<c_void>();
+        let result = ZSTD_rust_inBufferForEndFlush(ZSTD_BM_STABLE, expected_src, 37, 11);
+
+        assert_eq!(result.src, expected_src);
+        assert_eq!(result.size, 37);
+        assert_eq!(result.pos, 11);
+
+        let null_result = ZSTD_rust_inBufferForEndFlush(ZSTD_BM_STABLE, ptr::null(), 37, 11);
+
+        assert!(null_result.src.is_null());
+        assert_eq!(null_result.size, 37);
+        assert_eq!(null_result.pos, 11);
+    }
+
+    #[test]
+    fn in_buffer_for_end_flush_clears_buffered_and_other_modes() {
+        let expected_src = b"input".as_ptr().cast::<c_void>();
+
+        for mode in [ZSTD_BM_BUFFERED, 42] {
+            let result = ZSTD_rust_inBufferForEndFlush(mode, expected_src, 37, 11);
+
+            assert!(result.src.is_null());
+            assert_eq!(result.size, 0);
+            assert_eq!(result.pos, 0);
+        }
+    }
+
     #[test]
     fn invalidate_rep_codes_clears_all_entries() {
         let mut rep = [11u32, 22, 33];