feat(decompress): call dictionary entropy loader from Rust

The DDict implementation already owned the decoder-table loader in
zstd_decompress.rs, but zstd_ddict.rs still declared it as an external C
helper and disabled that path in Rust unit-test builds. Call the Rust
implementation directly through the ABI-compatible entropy-table
projection and make the module documentation match the actual ownership.
Keep offset_of imported only for the non-test C-context projection so the
Rust test check remains warning-free.

Test Plan:
- git diff --check
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo check --manifest-path rust/Cargo.toml --tests
- Standalone cargo test was attempted; linking remains blocked by the
  existing C context-projection symbols used by the hybrid test crate.

rust/src/zstd_ddict.rs

@@ -2,17 +2,17 @@
 
 //! Digested decompression dictionaries (`ZSTD_DDict`).
 //!
-//! The decoder proper is still implemented in C during the migration.  In
-//! particular, its `ZSTD_loadDEntropy()` helper owns the format-specific
-//! construction of the decoder tables.  This module owns the DDict object,
-//! its allocation and lifetime, and passes an ABI-identical table layout to
-//! that helper.
+//! The decoder proper and its `ZSTD_loadDEntropy()` dictionary-table loader
+//! are Rust-owned.  This module owns the DDict object, its allocation and
+//! lifetime, and passes an ABI-identical table layout to the decoder module.
 
 use crate::common::{LL_FSE_LOG, ML_FSE_LOG, OFF_FSE_LOG, ZSTD_FRAMEIDSIZE, ZSTD_REP_NUM};
 use crate::errors::{ERR_isError, ZstdErrorCode, ERROR};
 use crate::mem::MEM_readLE32;
 use std::ffi::c_void;
-use std::mem::{offset_of, size_of};
+#[cfg(not(test))]
+use std::mem::offset_of;
+use std::mem::size_of;
 use std::os::raw::{c_int, c_uint};
 use std::ptr;
 
@@ -103,15 +103,6 @@ struct ZstdDctxOffsets {
     fuzz_end: usize,
 }
 
-#[cfg(not(test))]
-unsafe extern "C" {
-    fn ZSTD_loadDEntropy(
-        entropy: *mut ZSTD_entropyDTables,
-        dict: *const c_void,
-        dict_size: usize,
-    ) -> usize;
-}
-
 #[cfg(test)]
 #[inline]
 unsafe fn ddict_dctx_offsets() -> ZstdDctxOffsets {
@@ -258,20 +249,16 @@ unsafe fn load_d_entropy(
     dict: *const c_void,
     dict_size: usize,
 ) -> usize {
-    #[cfg(not(test))]
-    {
-        // The existing C decoder owns this helper until its translation unit is
-        // moved.  Its argument is the ABI-compatible table storage above.
-        unsafe { ZSTD_loadDEntropy(entropy, dict, dict_size) }
-    }
-
-    #[cfg(test)]
-    {
-        let _ = (entropy, dict, dict_size);
-        // Unit tests exercise the DDict ownership and raw-content paths.  A
-        // full dictionary is covered by the C compatibility executables, where
-        // the decoder's real ZSTD_loadDEntropy() is linked in.
-        ERROR(ZstdErrorCode::DictionaryCorrupted)
+    // The DDict and decoder modules keep separate Rust representations so
+    // neither needs to expose the other module's private fields.  Both are
+    // `repr(C)` and guarded by exact layout assertions; only the pointer type
+    // changes at this narrow module boundary.
+    unsafe {
+        crate::zstd_decompress::ZSTD_loadDEntropy(
+            entropy.cast::<crate::zstd_decompress::ZSTD_entropyDTables_t>(),
+            dict,
+            dict_size,
+        )
     }
 }