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

Move ZSTD_initCStream's public initialization policy into the Rust
projection. Rust now preserves the reset, CDict-clear, and compression-level
ordering while C retains the private context callbacks and ABI boundary.

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

rust/src/zstd_compress.rs

@@ -1044,6 +1044,48 @@ pub unsafe extern "C" fn ZSTD_rust_initCStreamSrcSize(
     0
 }
 
+/// Explicit projection for `ZSTD_initCStream`.
+#[repr(C)]
+pub struct ZSTD_rust_initCStreamState {
+    callback_context: *mut c_void,
+    reset_session: InitCStreamUsingCDictAdvancedResetFn,
+    ref_cdict: InitCStreamUsingCDictAdvancedRefCDictFn,
+    set_level: InitCStreamSrcSizeSetLevelFn,
+}
+
+const _: () = {
+    assert!(offset_of!(ZSTD_rust_initCStreamState, callback_context) == 0);
+    assert!(offset_of!(ZSTD_rust_initCStreamState, reset_session) == size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_initCStreamState, ref_cdict) == 2 * size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_initCStreamState, set_level) == 3 * size_of::<usize>());
+    assert!(size_of::<ZSTD_rust_initCStreamState>() == 4 * size_of::<usize>());
+};
+
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_initCStream(
+    state: *const ZSTD_rust_initCStreamState,
+    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.ref_cdict)(state.callback_context, ptr::null()) };
+    if ERR_isError(result) {
+        return result;
+    }
+    let result = unsafe { (state.set_level)(state.callback_context, compression_level) };
+    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;
@@ -9162,6 +9204,72 @@ mod tests {
         assert_eq!(context.events, ["reset", "ref-cdict", "level", "pledged"]);
     }
 
+    fn init_cstream_test_state(
+        context: &mut InitCStreamUsingCDictAdvancedTestContext,
+    ) -> ZSTD_rust_initCStreamState {
+        ZSTD_rust_initCStreamState {
+            callback_context: (context as *mut InitCStreamUsingCDictAdvancedTestContext).cast(),
+            reset_session: init_cstream_using_cdict_advanced_test_reset,
+            ref_cdict: init_cstream_using_cdict_advanced_test_ref_cdict,
+            set_level: init_cstream_src_size_test_set_level,
+        }
+    }
+
+    #[test]
+    fn init_cstream_preserves_reset_clear_and_level_order() {
+        let mut context = InitCStreamUsingCDictAdvancedTestContext::default();
+        let state = init_cstream_test_state(&mut context);
+
+        let result = unsafe { ZSTD_rust_initCStream(&state, -3) };
+
+        assert_eq!(result, 0);
+        assert_eq!(context.events, ["reset", "ref-cdict", "level"]);
+        assert_eq!(context.cdict, ptr::null());
+        assert_eq!(context.compression_level, -3);
+    }
+
+    #[test]
+    fn init_cstream_stops_after_reset_error() {
+        let mut context = InitCStreamUsingCDictAdvancedTestContext {
+            reset_result: ERROR(ZstdErrorCode::MemoryAllocation),
+            ..InitCStreamUsingCDictAdvancedTestContext::default()
+        };
+        let state = init_cstream_test_state(&mut context);
+
+        let result = unsafe { ZSTD_rust_initCStream(&state, 4) };
+
+        assert_eq!(result, ERROR(ZstdErrorCode::MemoryAllocation));
+        assert_eq!(context.events, ["reset"]);
+    }
+
+    #[test]
+    fn init_cstream_stops_after_cdict_clear_error() {
+        let mut context = InitCStreamUsingCDictAdvancedTestContext {
+            ref_result: ERROR(ZstdErrorCode::DictionaryCreationFailed),
+            ..InitCStreamUsingCDictAdvancedTestContext::default()
+        };
+        let state = init_cstream_test_state(&mut context);
+
+        let result = unsafe { ZSTD_rust_initCStream(&state, 4) };
+
+        assert_eq!(result, ERROR(ZstdErrorCode::DictionaryCreationFailed));
+        assert_eq!(context.events, ["reset", "ref-cdict"]);
+    }
+
+    #[test]
+    fn init_cstream_propagates_level_error_last() {
+        let mut context = InitCStreamUsingCDictAdvancedTestContext {
+            set_level_result: ERROR(ZstdErrorCode::ParameterOutOfBound),
+            ..InitCStreamUsingCDictAdvancedTestContext::default()
+        };
+        let state = init_cstream_test_state(&mut context);
+
+        let result = unsafe { ZSTD_rust_initCStream(&state, 4) };
+
+        assert_eq!(result, ERROR(ZstdErrorCode::ParameterOutOfBound));
+        assert_eq!(context.events, ["reset", "ref-cdict", "level"]);
+    }
+
     #[test]
     fn pledged_src_size_writes_the_init_stage_value_plus_one() {
         let mut pledged_src_size_plus_one = 0;