refactor(compress): move stream buffer policy to Rust

Project the public ZSTD_compressStream2 buffer positions into Rust and
preserve the original output-first classification. C retains the
dstSize_tooSmall and srcSize_wrong mappings, exact diagnostics, private
context, and streaming state machine.

Test Plan:
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings` -- passed
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets -- -D warnings` -- passed
- `ulimit -v 41943040; cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check` -- passed
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 make -j1` -- passed
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 ./tests/rustLibSmoke` -- passed
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 make -j1 -C tests test` -- passed, including large streaming, native, fuzzer, and zstream phases

rust/src/zstd_compress.rs

@@ -164,6 +164,61 @@ const ZSTD_E_END: c_int = 2;
 const ZSTD_E_CONTINUE: c_int = 0;
 const ZSTD_E_FLUSH: c_int = 1;
 
+const ZSTD_RUST_COMPRESS_STREAM2_BUFFER_VALID: c_int = 0;
+const ZSTD_RUST_COMPRESS_STREAM2_BUFFER_OUTPUT_INVALID: c_int = 1;
+const ZSTD_RUST_COMPRESS_STREAM2_BUFFER_INPUT_INVALID: c_int = 2;
+
+/// Scalar projection for the public `ZSTD_compressStream2` buffer preconditions.
+///
+/// C retains the public error mappings and diagnostics, while Rust owns the
+/// output-first classification of the scalar position/size pairs.  The C
+/// context and streaming state machine remain outside this projection.
+#[repr(C)]
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+pub struct ZSTD_rust_compressStream2BufferPolicyState {
+    pub output_pos: usize,
+    pub output_size: usize,
+    pub input_pos: usize,
+    pub input_size: usize,
+}
+
+const _: () = {
+    assert!(offset_of!(ZSTD_rust_compressStream2BufferPolicyState, output_pos) == 0);
+    assert!(
+        offset_of!(ZSTD_rust_compressStream2BufferPolicyState, output_size) == size_of::<usize>()
+    );
+    assert!(
+        offset_of!(ZSTD_rust_compressStream2BufferPolicyState, input_pos) == 2 * size_of::<usize>()
+    );
+    assert!(
+        offset_of!(ZSTD_rust_compressStream2BufferPolicyState, input_size)
+            == 3 * size_of::<usize>()
+    );
+    assert!(size_of::<ZSTD_rust_compressStream2BufferPolicyState>() == 4 * size_of::<usize>());
+};
+
+#[inline]
+fn compress_stream2_buffer_policy(state: &ZSTD_rust_compressStream2BufferPolicyState) -> c_int {
+    if state.output_pos > state.output_size {
+        return ZSTD_RUST_COMPRESS_STREAM2_BUFFER_OUTPUT_INVALID;
+    }
+    if state.input_pos > state.input_size {
+        return ZSTD_RUST_COMPRESS_STREAM2_BUFFER_INPUT_INVALID;
+    }
+    ZSTD_RUST_COMPRESS_STREAM2_BUFFER_VALID
+}
+
+/// Classify stream buffer positions before C enters its private state machine.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_compressStream2BufferPolicy(
+    state: *const ZSTD_rust_compressStream2BufferPolicyState,
+) -> c_int {
+    let Some(state) = (unsafe { state.as_ref() }) else {
+        return ZSTD_RUST_COMPRESS_STREAM2_BUFFER_OUTPUT_INVALID;
+    };
+    compress_stream2_buffer_policy(state)
+}
+
 /// Scalar projection for the public `ZSTD_compressStream2` directive policy.
 ///
 /// The C fallback retains the private context and streaming implementation;
@@ -14071,6 +14126,48 @@ mod tests {
         assert_eq!(context.calls, 0);
     }
 
+    #[test]
+    fn compress_stream2_buffer_policy_accepts_valid_positions() {
+        let state = ZSTD_rust_compressStream2BufferPolicyState {
+            output_pos: 3,
+            output_size: 3,
+            input_pos: 7,
+            input_size: 7,
+        };
+        assert_eq!(
+            compress_stream2_buffer_policy(&state),
+            ZSTD_RUST_COMPRESS_STREAM2_BUFFER_VALID
+        );
+    }
+
+    #[test]
+    fn compress_stream2_buffer_policy_rejects_invalid_output_positions() {
+        let state = ZSTD_rust_compressStream2BufferPolicyState {
+            output_pos: 4,
+            output_size: 3,
+            input_pos: 7,
+            input_size: 7,
+        };
+        assert_eq!(
+            compress_stream2_buffer_policy(&state),
+            ZSTD_RUST_COMPRESS_STREAM2_BUFFER_OUTPUT_INVALID
+        );
+    }
+
+    #[test]
+    fn compress_stream2_buffer_policy_rejects_invalid_input_positions() {
+        let state = ZSTD_rust_compressStream2BufferPolicyState {
+            output_pos: 3,
+            output_size: 3,
+            input_pos: 8,
+            input_size: 7,
+        };
+        assert_eq!(
+            compress_stream2_buffer_policy(&state),
+            ZSTD_RUST_COMPRESS_STREAM2_BUFFER_INPUT_INVALID
+        );
+    }
+
     #[test]
     fn compress_stream2_policy_accepts_all_public_directives() {
         for end_op in [ZSTD_E_CONTINUE, ZSTD_E_FLUSH, ZSTD_E_END] {