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/README.md

@@ -0,0 +1,57 @@
+# Rust rewrite
+
+This directory contains the in-progress Rust replacement for the zstd library
+and command-line program. During the migration, the crate is built as a static
+library and linked into the original C test programs. Production C translation
+units become declaration-only shims as their implementations move to Rust; the
+original C tests remain unchanged and provide compatibility coverage.
+
+## Component map
+
+The crate is organized from low-level representation helpers toward the public
+zstd ABI:
+
+- Common primitives
+  - `mem`, `bits`, `bitstream`, and `cpu` implement byte-order, bitstream, and
+    target-feature operations used by the codecs.
+  - `errors`, `debug`, `xxhash`, and `zstd_common` provide common exported ABI
+    functions and state.
+  - `common` contains shared frame constants and internal data types.
+All entropy coding, compression, decompression, dictionary, legacy, runtime,
+and CLI translation units are still C at this initial stage. 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
+
+The public ABI continues to come from the existing headers under `lib/`.
+Exported Rust functions therefore use C layout and calling conventions. A C
+source file whose implementation has moved to Rust remains in the original
+makefile source list as a small shim so header configuration and platform
+preprocessor behavior stay available during the transition.
+
+The original test makefile links `target/release/libzstd_rs.a` with whole-archive
+semantics. Rebuild the Rust archive before running C tests so the executable
+does not use a stale implementation.
+
+## Validation
+
+Run focused Rust checks from this directory:
+
+```sh
+cargo fmt --check
+cargo clippy --all-targets -- -D warnings
+cargo test --all-targets
+cargo build --release
+```
+
+Then run original compatibility tests from the repository root, starting with
+the narrow target for the component being migrated. For example:
+
+```sh
+make -C tests fuzzer
+./tests/fuzzer -i1 --no-big-tests
+```
+
+Broader `tests/Makefile` targets remain the authoritative integration gates as
+more of the library and CLI are rewritten.