feat(compress): move dictionary stream init policy into Rust

Move ZSTD_initCStream_usingDict's public initialization policy into the Rust
projection. Rust now preserves reset, compression-level, and dictionary-load
ordering while C retains the private context and dictionary callbacks.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml init_cstream_using_dict -- --test-threads=1
- ulimit -v 41943040; make -B -C programs -j1 zstd
- git diff --cached --check

lib/compress/zstd_compress.c

@@ -148,6 +148,25 @@ typedef char ZSTD_rust_init_cstream_state_layout[
          && offsetof(ZSTD_rust_initCStreamState, setLevel) == 3 * sizeof(void*)
          && sizeof(ZSTD_rust_initCStreamState) == 4 * sizeof(void*))
         ? 1 : -1];
+typedef size_t (*ZSTD_rust_initCStreamUsingDictLoadDictionary_f)(
+        void* context, const void* dict, size_t dictSize);
+typedef struct {
+    void* callbackContext;
+    ZSTD_rust_initCStreamUsingCDictAdvancedReset_f resetSession;
+    ZSTD_rust_initCStreamSrcSizeSetLevel_f setLevel;
+    ZSTD_rust_initCStreamUsingDictLoadDictionary_f loadDictionary;
+} ZSTD_rust_initCStreamUsingDictState;
+size_t ZSTD_rust_initCStreamUsingDict(
+        const ZSTD_rust_initCStreamUsingDictState* state,
+        const void* dict, size_t dictSize, int compressionLevel);
+typedef char ZSTD_rust_init_cstream_using_dict_state_layout[
+        (offsetof(ZSTD_rust_initCStreamUsingDictState, callbackContext) == 0
+         && offsetof(ZSTD_rust_initCStreamUsingDictState, resetSession) == sizeof(void*)
+         && offsetof(ZSTD_rust_initCStreamUsingDictState, setLevel) == 2 * sizeof(void*)
+         && offsetof(ZSTD_rust_initCStreamUsingDictState, loadDictionary)
+                == 3 * sizeof(void*)
+         && sizeof(ZSTD_rust_initCStreamUsingDictState) == 4 * sizeof(void*))
+        ? 1 : -1];
 ZSTD_frameProgression ZSTD_rust_frameProgression(U64 consumedSrcSize,
                                                   size_t buffered,
                                                   U64 producedCSize);
@@ -4991,15 +5010,6 @@ size_t ZSTD_initCStream_advanced(ZSTD_CStream* zcs,
     return 0;
 }
 
-size_t ZSTD_initCStream_usingDict(ZSTD_CStream* zcs, const void* dict, size_t dictSize, int compressionLevel)
-{
-    DEBUGLOG(4, "ZSTD_initCStream_usingDict");
-    FORWARD_IF_ERROR( ZSTD_CCtx_reset(zcs, ZSTD_reset_session_only) , "");
-    FORWARD_IF_ERROR( ZSTD_CCtx_setParameter(zcs, ZSTD_c_compressionLevel, compressionLevel) , "");
-    FORWARD_IF_ERROR( ZSTD_CCtx_loadDictionary(zcs, dict, dictSize) , "");
-    return 0;
-}
-
 static size_t ZSTD_rust_initCStreamSrcSize_setLevel(
         void* context, int compressionLevel)
 {
@@ -5007,6 +5017,24 @@ static size_t ZSTD_rust_initCStreamSrcSize_setLevel(
             (ZSTD_CCtx*)context, ZSTD_c_compressionLevel, compressionLevel);
 }
 
+static size_t ZSTD_rust_initCStreamUsingDict_loadDictionary(
+        void* context, const void* dict, size_t dictSize)
+{
+    return ZSTD_CCtx_loadDictionary((ZSTD_CCtx*)context, dict, dictSize);
+}
+
+size_t ZSTD_initCStream_usingDict(ZSTD_CStream* zcs, const void* dict, size_t dictSize, int compressionLevel)
+{
+    ZSTD_rust_initCStreamUsingDictState state;
+    DEBUGLOG(4, "ZSTD_initCStream_usingDict");
+    state.callbackContext = zcs;
+    state.resetSession = ZSTD_rust_initCStreamUsingCDictAdvanced_resetSession;
+    state.setLevel = ZSTD_rust_initCStreamSrcSize_setLevel;
+    state.loadDictionary = ZSTD_rust_initCStreamUsingDict_loadDictionary;
+    return ZSTD_rust_initCStreamUsingDict(
+            &state, dict, dictSize, compressionLevel);
+}
+
 size_t ZSTD_initCStream_srcSize(ZSTD_CStream* zcs, int compressionLevel, unsigned long long pss)
 {
     ZSTD_rust_initCStreamSrcSizeState state;

rust/src/zstd_compress.rs

@@ -1086,6 +1086,55 @@ pub unsafe extern "C" fn ZSTD_rust_initCStream(
     0
 }
 
+type InitCStreamUsingDictLoadDictionaryFn =
+    unsafe extern "C" fn(*mut c_void, *const c_void, usize) -> usize;
+
+/// Explicit projection for `ZSTD_initCStream_usingDict`.
+#[repr(C)]
+pub struct ZSTD_rust_initCStreamUsingDictState {
+    callback_context: *mut c_void,
+    reset_session: InitCStreamUsingCDictAdvancedResetFn,
+    set_level: InitCStreamSrcSizeSetLevelFn,
+    load_dictionary: InitCStreamUsingDictLoadDictionaryFn,
+}
+
+const _: () = {
+    assert!(offset_of!(ZSTD_rust_initCStreamUsingDictState, callback_context) == 0);
+    assert!(offset_of!(ZSTD_rust_initCStreamUsingDictState, reset_session) == size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_initCStreamUsingDictState, set_level) == 2 * size_of::<usize>());
+    assert!(
+        offset_of!(ZSTD_rust_initCStreamUsingDictState, load_dictionary) == 3 * size_of::<usize>()
+    );
+    assert!(size_of::<ZSTD_rust_initCStreamUsingDictState>() == 4 * size_of::<usize>());
+};
+
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_initCStreamUsingDict(
+    state: *const ZSTD_rust_initCStreamUsingDictState,
+    dict: *const c_void,
+    dict_size: usize,
+    compression_level: c_int,
+) -> usize {
+    if state.is_null() {
+        return ERROR(ZstdErrorCode::Generic);
+    }
+    let state = unsafe { &*state };
+
+    let result = unsafe { (state.reset_session)(state.callback_context) };
+    if ERR_isError(result) {
+        return result;
+    }
+    let result = unsafe { (state.set_level)(state.callback_context, compression_level) };
+    if ERR_isError(result) {
+        return result;
+    }
+    let result = unsafe { (state.load_dictionary)(state.callback_context, dict, dict_size) };
+    if ERR_isError(result) {
+        return result;
+    }
+    0
+}
+
 type CompressStreamBlockFn =
     unsafe extern "C" fn(*mut c_void, *mut c_void, usize, *const c_void, usize) -> usize;
 type CompressStreamResetFn = unsafe extern "C" fn(*mut c_void) -> usize;
@@ -8936,11 +8985,13 @@ mod tests {
         reset_result: usize,
         pledged_result: usize,
         set_level_result: usize,
+        load_dict_result: usize,
         ref_result: usize,
         pledged_src_size: u64,
         compression_level: c_int,
         frame_params: [c_uint; 3],
         cdict: *const c_void,
+        dict_size: usize,
     }
 
     unsafe fn init_cstream_using_cdict_advanced_test_context(
@@ -8977,6 +9028,18 @@ mod tests {
         context.set_level_result
     }
 
+    unsafe extern "C" fn init_cstream_using_dict_test_load_dictionary(
+        context: *mut c_void,
+        dict: *const c_void,
+        dict_size: usize,
+    ) -> usize {
+        let context = unsafe { init_cstream_using_cdict_advanced_test_context(context) };
+        context.events.push("load-dict");
+        context.cdict = dict;
+        context.dict_size = dict_size;
+        context.load_dict_result
+    }
+
     unsafe extern "C" fn init_cstream_using_cdict_advanced_test_set_frame_params(
         context: *mut c_void,
         content_size_flag: c_uint,
@@ -9270,6 +9333,74 @@ mod tests {
         assert_eq!(context.events, ["reset", "ref-cdict", "level"]);
     }
 
+    fn init_cstream_using_dict_test_state(
+        context: &mut InitCStreamUsingCDictAdvancedTestContext,
+    ) -> ZSTD_rust_initCStreamUsingDictState {
+        ZSTD_rust_initCStreamUsingDictState {
+            callback_context: (context as *mut InitCStreamUsingCDictAdvancedTestContext).cast(),
+            reset_session: init_cstream_using_cdict_advanced_test_reset,
+            set_level: init_cstream_src_size_test_set_level,
+            load_dictionary: init_cstream_using_dict_test_load_dictionary,
+        }
+    }
+
+    #[test]
+    fn init_cstream_using_dict_preserves_order_and_dictionary_arguments() {
+        let dict = ptr::dangling::<c_void>();
+        let mut context = InitCStreamUsingCDictAdvancedTestContext::default();
+        let state = init_cstream_using_dict_test_state(&mut context);
+
+        let result = unsafe { ZSTD_rust_initCStreamUsingDict(&state, dict, 123, -3) };
+
+        assert_eq!(result, 0);
+        assert_eq!(context.events, ["reset", "level", "load-dict"]);
+        assert_eq!(context.compression_level, -3);
+        assert_eq!(context.cdict, dict);
+        assert_eq!(context.dict_size, 123);
+    }
+
+    #[test]
+    fn init_cstream_using_dict_stops_after_reset_error() {
+        let mut context = InitCStreamUsingCDictAdvancedTestContext {
+            reset_result: ERROR(ZstdErrorCode::MemoryAllocation),
+            ..InitCStreamUsingCDictAdvancedTestContext::default()
+        };
+        let state = init_cstream_using_dict_test_state(&mut context);
+
+        let result = unsafe { ZSTD_rust_initCStreamUsingDict(&state, ptr::null(), 0, 4) };
+
+        assert_eq!(result, ERROR(ZstdErrorCode::MemoryAllocation));
+        assert_eq!(context.events, ["reset"]);
+    }
+
+    #[test]
+    fn init_cstream_using_dict_stops_after_level_error() {
+        let mut context = InitCStreamUsingCDictAdvancedTestContext {
+            set_level_result: ERROR(ZstdErrorCode::ParameterOutOfBound),
+            ..InitCStreamUsingCDictAdvancedTestContext::default()
+        };
+        let state = init_cstream_using_dict_test_state(&mut context);
+
+        let result = unsafe { ZSTD_rust_initCStreamUsingDict(&state, ptr::null(), 0, 4) };
+
+        assert_eq!(result, ERROR(ZstdErrorCode::ParameterOutOfBound));
+        assert_eq!(context.events, ["reset", "level"]);
+    }
+
+    #[test]
+    fn init_cstream_using_dict_propagates_dictionary_error_last() {
+        let mut context = InitCStreamUsingCDictAdvancedTestContext {
+            load_dict_result: ERROR(ZstdErrorCode::DictionaryCreationFailed),
+            ..InitCStreamUsingCDictAdvancedTestContext::default()
+        };
+        let state = init_cstream_using_dict_test_state(&mut context);
+
+        let result = unsafe { ZSTD_rust_initCStreamUsingDict(&state, ptr::null(), 0, 4) };
+
+        assert_eq!(result, ERROR(ZstdErrorCode::DictionaryCreationFailed));
+        assert_eq!(context.events, ["reset", "level", "load-dict"]);
+    }
+
     #[test]
     fn pledged_src_size_writes_the_init_stage_value_plus_one() {
         let mut pledged_src_size_plus_one = 0;