feat(dict): port the public zdict builder to Rust

Move the public ZDICT helpers, legacy trainer, dictionary finalization,
entropy-table construction, and supporting dictionary logic into Rust. Keep
the C translation unit as an ABI anchor and register the Rust module only
when compression and dictionary-builder features are enabled.

Test Plan:
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= cargo test --manifest-path rust/Cargo.toml dict_builder_zdict
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- rustfmt +nightly --check --edition 2021 rust/src/dict_builder_zdict.rs rust/src/lib.rs
- git diff --cached --check

rust/src/lib.rs

@@ -6,6 +6,10 @@ pub mod common;
 pub mod cpu;
 pub mod debug;
 #[cfg(feature = "dict-builder")]
+pub mod dict_builder_cover;
+#[cfg(all(feature = "compression", feature = "dict-builder"))]
+pub mod dict_builder_zdict;
+#[cfg(feature = "dict-builder")]
 pub mod divsufsort;
 pub mod entropy_common;
 pub mod errors;
@@ -18,8 +22,6 @@ pub mod hist;
 pub mod huf_compress;
 #[cfg(feature = "decompression")]
 pub mod huf_decompress;
-#[cfg(feature = "dict-builder")]
-pub mod dict_builder_cover;
 pub mod legacy;
 pub mod mem;
 pub mod pool;