Replace the v0.5 and v0.6 legacy decoder translation units with Rust modules
and keep only narrow C registration shims for the public legacy dispatch ABI.
Test Plan:
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= cargo test --manifest-path rust/Cargo.toml --features compression,decompression,dict-builder,legacy-v05,legacy-v06,legacy-v07
- RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= cargo clippy --manifest-path rust/Cargo.toml --all-targets --features compression,decompression,dict-builder,legacy-v05,legacy-v06,legacy-v07 -- -D warnings
- make -C tests check V=1