refactor(decompress): remove redundant dictionary bridge

Call the authoritative DDict constructor directly from the Rust decompression
policy after converting the shared repr(C) allocator projection.  Remove the
C forwarding declaration and wrapper so dictionary creation no longer crosses
an unnecessary Rust-to-C-to-C boundary, while compile-time size and alignment
checks keep the allocator conversion tied to the ABI contract.

Test Plan:
- `ulimit -v 41943040; cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check`
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings`
- `ulimit -v 41943040; make -j1`
- `ulimit -v 41943040; make -j1 -C tests test`
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets -- -D warnings`
- `ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/cli/Cargo.toml --all-targets`

The integrated checks ran at the combined working-tree tip, including the
parallel MT checksum seam that remains as the next commit. Standalone root
Rust unit linking remains unavailable because the crate imports C-owned bridge
symbols without a Cargo build/link setup.

lib/decompress/zstd_decompress.c

@@ -142,10 +142,6 @@ size_t ZSTD_rust_decompress_stack(void* dst, size_t dstCapacity,
 size_t ZSTD_rust_decompress_stack_context(ZSTD_DCtx* dctx,
                                           void* dst, size_t dstCapacity,
                                           const void* src, size_t srcSize);
-ZSTD_DDict* ZSTD_rust_create_ddict(const void* dict, size_t dictSize,
-                                    ZSTD_dictLoadMethod_e dictLoadMethod,
-                                    ZSTD_dictContentType_e dictContentType,
-                                    ZSTD_customMem customMem);
 unsigned ZSTD_rust_legacy_support(void);
 
 #if ZSTD_TRACE
@@ -291,15 +287,6 @@ size_t ZSTD_rust_decompress_stack(void* dst, size_t dstCapacity,
 #endif
 }
 
-ZSTD_DDict* ZSTD_rust_create_ddict(const void* dict, size_t dictSize,
-                                    ZSTD_dictLoadMethod_e dictLoadMethod,
-                                    ZSTD_dictContentType_e dictContentType,
-                                    ZSTD_customMem customMem)
-{
-    return ZSTD_createDDict_advanced(dict, dictSize, dictLoadMethod,
-                                     dictContentType, customMem);
-}
-
 void ZSTD_rust_dctx_trace_view(ZSTD_DCtx* dctx,
                                ZSTD_rustDctxTraceView* out)
 {

rust/src/zstd_decompress.rs

@@ -22,7 +22,7 @@ use crate::mem::{MEM_32bits, MEM_readLE16, MEM_readLE32, MEM_readLE64};
 use crate::xxhash::{XXH64_digest, XXH64_reset, XXH64_state_t, XXH64_update, XXH64};
 use crate::zstd_ddict::{
     ZSTD_DDict, ZSTD_DDict_dictContent, ZSTD_DDict_dictSize, ZSTD_copyDDictParameters,
-    ZSTD_freeDDict, ZSTD_getDictID_fromDDict,
+    ZSTD_createDDict_advanced, ZSTD_freeDDict, ZSTD_getDictID_fromDDict,
 };
 use std::cmp::{max, min};
 use std::ffi::c_void;
@@ -304,6 +304,8 @@ const _: () = {
     assert!(offset_of!(ZSTD_rustDctxView, static_size) == 29 * size_of::<usize>());
     assert!(offset_of!(ZSTD_rustDctxView, dctx_size) == 67 * size_of::<usize>());
     assert!(size_of::<ZSTD_rustDctxView>() == 68 * size_of::<usize>());
+    assert!(size_of::<ZSTD_customMem>() == size_of::<crate::zstd_ddict::ZSTD_customMem>());
+    assert!(align_of::<ZSTD_customMem>() == align_of::<crate::zstd_ddict::ZSTD_customMem>());
     assert!(align_of::<ZSTD_rustDctxView>() == align_of::<usize>());
 };
 
@@ -352,13 +354,6 @@ unsafe extern "C" {
         src: *const c_void,
         src_size: usize,
     ) -> usize;
-    fn ZSTD_rust_create_ddict(
-        dict: *const c_void,
-        dict_size: usize,
-        dict_load_method: c_int,
-        dict_content_type: c_int,
-        custom_mem: ZSTD_customMem,
-    ) -> *mut ZSTD_DDict;
     #[cfg(all(
         not(test),
         any(
@@ -1719,6 +1714,11 @@ unsafe fn dctx_custom_mem(view: &ZSTD_rustDctxView) -> ZSTD_customMem {
     unsafe { field(view.custom_mem) }
 }
 
+#[inline]
+unsafe fn ddict_custom_mem(view: &ZSTD_rustDctxView) -> crate::zstd_ddict::ZSTD_customMem {
+    unsafe { std::mem::transmute(dctx_custom_mem(view)) }
+}
+
 #[inline]
 unsafe fn dctx_ddict(view: &ZSTD_rustDctxView) -> *const ZSTD_DDict {
     unsafe { field(view.ddict) }
@@ -3991,12 +3991,12 @@ pub unsafe extern "C" fn ZSTD_DCtx_loadDictionary_advanced(
     unsafe { clear_dict(&view) };
     if !dict.is_null() && dict_size != 0 {
         let ddict = unsafe {
-            ZSTD_rust_create_ddict(
+            ZSTD_createDDict_advanced(
                 dict,
                 dict_size,
                 dict_load_method,
                 dict_content_type,
-                dctx_custom_mem(&view),
+                ddict_custom_mem(&view),
             )
         };
         if ddict.is_null() {