feat(rust): port decode dictionaries

Move ZSTD_DDict allocation, ownership, construction, and public ABI exports
into Rust. The C decoder keeps ZSTD_loadDEntropy until its table loader moves.

The C shim now exposes C-computed DCtx field offsets. This keeps dictionary
state correct across conditional C layouts, including fuzz fields and 32-bit
static-BMI2 builds, without duplicating decoder-context layout in Rust.

Test Plan:
- cargo fmt, cargo clippy, cargo clippy --benches, cargo clippy --tests
- cargo test --all-targets and cargo build --release
- cargo test/build --target i686-unknown-linux-gnu
- C fuzzer, zstreamtest, invalidDictionaries, and fuzzer32 smoke runs
- fuzz-enabled and i686 -mbmi2 C/Rust dictionary roundtrip harnesses

Refs: rust/README.md

rust/README.md

@@ -29,11 +29,13 @@ zstd ABI:
 - Runtime support
   - `threading` provides platform pthread wrappers required by zstd headers.
   - `pool` implements the bounded worker pool used by multithreaded compression.
+- Dictionary support
+  - `zstd_ddict` owns, loads, copies, and references decode dictionaries.
 
-The remaining block compression, general decompression, dictionary, legacy,
-and CLI translation units are still C. They must move before the rewrite is
-complete. Keeping that boundary explicit prevents a passing hybrid build from
-being mistaken for the final all-Rust result.
+The remaining block compression, general decompression, dictionary-building,
+legacy, and CLI translation units are still C. They must move before the
+rewrite is complete. Keeping that boundary explicit prevents a passing hybrid
+build from being mistaken for the final all-Rust result.
 
 ## Compatibility boundary