feat(rust): port double-fast block matching

Move the two-table fast match finder into Rust while retaining a small C
projection of the private match-state. The Rust implementation handles normal,
attached-dictionary, and external-dictionary matching without exposing the
full C context ABI. The C entry points and build-time exclusion guards remain
unchanged for configured consumers.

Document the migrated matcher in the Rust component map and narrow the
remaining-C boundary accordingly.

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

@@ -31,13 +31,15 @@ zstd ABI:
   - `zstd_presplit` chooses split points for full compression blocks.
   - `zstd_compress_literals` emits raw, RLE, and Huffman literal sections
     while preserving the compressor's Huffman-table repeat state.
+  - `zstd_fast` and `zstd_double_fast` implement the single- and two-table
+    fast block match finders, including attached and external dictionary paths.
 - 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-building,
+The lazy and optimal block matchers, 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.