feat(rust): port compressed block decoding

Move literal decoding, sequence-table construction, FSE sequence decoding, and
sequence execution into Rust. The C shim retains ownership of the configured
decoder context and passes only the leaf state needed by the block codec.

Correct the two high offset-code bases while porting the tables. Their prior
values made valid large-window streams decode as corrupted data; the new unit
test fixes the exact values and a native zstream regression exercises them.
High-level frame and streaming context control remains C for now.

Test Plan:
- cargo test --all-targets
- cargo test --target i686-unknown-linux-gnu --all-targets
- cargo clippy && cargo clippy --benches && cargo clippy --tests
- cargo +nightly fmt
- make -B -C tests -j2 fuzzer zstreamtest invalidDictionaries poolTests
- ./tests/fuzzer -s5346 -i1 --no-big-tests
- ./tests/zstreamtest -i3000 -s334462
- ./tests/invalidDictionaries
- timeout 20s stdbuf -oL ./tests/poolTests

Refs: rust/README.md

rust/README.md

@@ -40,11 +40,14 @@ zstd ABI:
   - `pool` implements the bounded worker pool used by multithreaded compression.
 - Dictionary support
   - `zstd_ddict` owns, loads, copies, and references decode dictionaries.
+- Block decompression
+  - `zstd_decompress_block` decodes literal and sequence sections, maintains
+    FSE/Huffman repeat state, and executes compressed-block sequences.
 
-The optimal block matcher, 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.
+The optimal block matcher, high-level frame 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