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)
 {