feat(rust): add common primitive compatibility layer

Introduce the Rust static library and move the shared byte, bitstream, CPU,
error, xxHash, debug, and public-common implementations into it. Thin C shims
preserve the existing header-driven C build while original tests link the Rust
archive.

This establishes the ABI-safe foundation for later codec and CLI ports;
entropy coding, runtime support, codecs, dictionaries, and the CLI remain C.
The top-down migration map documents that boundary and its validation path.

Test Plan:
- cargo fmt --check
- cargo test --all-targets
- cargo clippy --all-targets -- -D warnings
- cargo build --release

Refs: rust/README.md

rust/src/lib.rs

@@ -0,0 +1,11 @@
+#![allow(clippy::missing_safety_doc)]
+
+pub mod bits;
+pub mod bitstream;
+pub mod common;
+pub mod cpu;
+pub mod debug;
+pub mod errors;
+pub mod mem;
+pub mod xxhash;
+pub mod zstd_common;