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

tests/Makefile

@@ -74,6 +74,7 @@ MULTITHREAD_CPP = -DZSTD_MULTITHREAD
 MULTITHREAD_LD  = -pthread
 endif
 MULTITHREAD = $(MULTITHREAD_CPP) $(MULTITHREAD_LD)
+LDFLAGS += -Wl,--whole-archive ../rust/target/release/libzstd_rs.a -Wl,--no-whole-archive
 
 VOID = /dev/null
 ZSTREAM_TESTTIME ?= -T90s