refactor(mt): move serial reset error policy to Rust

Project the private serial-state reset result into Rust for the stream-initialization error decision while retaining C-owned allocation, serial-state mutation, dictionary setup, synchronization, callback order, and ABI layouts. Nonzero callback status preserves the existing memory_allocation error path.

Test Plan: ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings; cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets -- -D warnings; cargo test --manifest-path rust/cli/Cargo.toml --all-targets (188 passed); cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check; make -j1; ./tests/rustLibSmoke; make -j1 -C tests test (all shell tests, large streaming tests, native tester, fuzzer phases, and zstream tester passed).

lib/compress/zstdmt_compress.c

@@ -853,6 +853,7 @@ size_t ZSTDMT_rust_initCStream(
         ZSTDMT_initResetStreamFn resetStream,
         ZSTDMT_initDictionaryFn updateDictionary,
         ZSTDMT_initSerialResetFn serialReset);
+size_t ZSTDMT_rust_initSerialResetResult(int status);
 typedef void (*ZSTDMT_resetStreamCallbackFn)(void* opaque);
 typedef struct {
     void* callbackContext;
@@ -2779,12 +2780,10 @@ static size_t ZSTDMT_initCStreamUpdateDictionary(void* opaque)
 static size_t ZSTDMT_initCStreamSerialReset(void* opaque, size_t targetSectionSize)
 {
     ZSTDMT_initCStreamState* const state = (ZSTDMT_initCStreamState*)opaque;
-    if (ZSTDMT_serialState_reset(
+    return ZSTDMT_rust_initSerialResetResult(ZSTDMT_serialState_reset(
             &state->mtctx->serial, state->mtctx->seqPool, state->params,
             targetSectionSize, state->dict, state->dictSize,
-            state->dictContentType))
+            state->dictContentType));
-        return ERROR(memory_allocation);
-    return 0;
 }
 
 /* ====================================== */

rust/src/zstdmt_compress.rs

@@ -2136,6 +2136,23 @@ fn normalize_mt_job_size(job_size: usize, job_size_min: usize, job_size_max: usi
     }
 }
 
+#[inline]
+fn init_serial_reset_result(status: c_int) -> usize {
+    if status != 0 {
+        ERROR(ZstdErrorCode::MemoryAllocation)
+    } else {
+        0
+    }
+}
+
+/// Map the private C serial-reset result to the MT stream-initialization ABI
+/// error. C retains serial-state allocation and mutation; Rust owns only this
+/// scalar error policy.
+#[no_mangle]
+pub extern "C" fn ZSTDMT_rust_initSerialResetResult(status: c_int) -> usize {
+    init_serial_reset_result(status)
+}
+
 #[inline]
 fn init_round_buffer_capacity(
     projection: ZSTDMT_initCStreamProjection,
@@ -6590,6 +6607,18 @@ mod tests {
         );
     }
 
+    #[test]
+    fn init_serial_reset_result_maps_nonzero_status_to_memory_error() {
+        let expected_error = ERROR(ZstdErrorCode::MemoryAllocation);
+
+        assert_eq!(init_serial_reset_result(0), 0);
+        assert_eq!(ZSTDMT_rust_initSerialResetResult(0), 0);
+        for status in [1, -1, 2] {
+            assert_eq!(init_serial_reset_result(status), expected_error);
+            assert_eq!(ZSTDMT_rust_initSerialResetResult(status), expected_error);
+        }
+    }
+
     #[test]
     fn init_c_stream_preserves_success_order_and_normalization() {
         let projection = init_projection();