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

lib/common/debug.c

@@ -1,30 +1,4 @@
-/* ******************************************************************
- * debug
- * Part of FSE library
- * Copyright (c) Meta Platforms, Inc. and affiliates.
- *
- * You can contact the author at :
- * - Source repository : https://github.com/Cyan4973/FiniteStateEntropy
- *
- * This source code is licensed under both the BSD-style license (found in the
- * LICENSE file in the root directory of this source tree) and the GPLv2 (found
- * in the COPYING file in the root directory of this source tree).
- * You may select, at your option, one of the above-listed licenses.
-****************************************************************** */
-
-
-/*
- * This module only hosts one global variable
- * which can be used to dynamically influence the verbosity of traces,
- * such as DEBUGLOG and RAWLOG
- */
-
 #include "debug.h"
 
-#if !defined(ZSTD_LINUX_KERNEL) || (DEBUGLEVEL>=2)
-/* We only use this when DEBUGLEVEL>=2, but we get -Werror=pedantic errors if a
- * translation unit is empty. So remove this from Linux kernel builds, but
- * otherwise just leave it in.
- */
-int g_debuglevel = DEBUGLEVEL;
-#endif
+/* Implementation moved to Rust (rust/src/debug.rs) */
+extern int g_debuglevel;