refactor(compress): expose dictionary entropy leaf from Rust

The dictionary entropy-header loader was already implemented in Rust, but C
kept a private bridge declaration and retained `ZSTD_loadCEntropy` as a
forwarding wrapper. Export the original ABI name from Rust and retain a
crate-local `ZSTD_rust_loadCEntropy` alias for existing Rust callers. Remove
only the bridge declaration and forwarding body; dictionary-content
orchestration, callbacks, and private C layouts remain unchanged.

Test Plan:
- `cc -std=c99 -Ilib -Ilib/common -Ilib/compress -Ilib/decompress -Ilib/dict -fsyntax-only lib/compress/zstd_compress.c` -- passed.
- `git diff --check` -- passed.
- `cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check` -- passed.
- `git diff --cached --check` -- passed.
- Cargo build/test, make, and other heavy checks were intentionally not run per
  the requested lightweight-only scope.

lib/compress/zstd_compress.c

@@ -2139,8 +2139,6 @@ void ZSTD_rust_storeLastLiterals(SeqStore_t* seqStorePtr,
 void ZSTD_rust_validateSeqStore(const SeqStore_t* seqStore, U32 minMatch);
 U32 ZSTD_rust_resolveRepcodeToRawOffset(const U32 rep[ZSTD_REP_NUM],
                                         U32 offBase, U32 ll0);
-size_t ZSTD_rust_loadCEntropy(ZSTD_compressedBlockState_t* bs, void* workspace,
-                              const void* dict, size_t dictSize);
 /* Rust owns dictionary-content orchestration and policy. The callbacks below
  * keep configuration-sensitive match-state, LDM, workspace, and matchfinder
  * operations private to C. */
@@ -6231,14 +6229,6 @@ static void ZSTD_loadDictionaryContent_assertInvalidStrategy(void* opaque)
 }
 
 
-/* Dictionary entropy-header loading and content orchestration live in Rust;
- * this keeps the original internal symbol available to C callers. */
-size_t ZSTD_loadCEntropy(ZSTD_compressedBlockState_t* bs, void* workspace,
-                         const void* const dict, size_t dictSize)
-{
-    return ZSTD_rust_loadCEntropy(bs, workspace, dict, dictSize);
-}
-
 /* Package only scalar policy inputs and opaque private-operation callbacks for
  * the Rust dictionary-content orchestrator. */
 static size_t ZSTD_loadDictionaryContent_callback(