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/debug.rs

@@ -0,0 +1,52 @@
+#![allow(non_snake_case)]
+
+/// Global debug verbosity level (C `g_debuglevel` from debug.c).
+/// Only meaningful when debug logging is enabled at compile time.
+#[no_mangle]
+pub static mut g_debuglevel: i32 = 0;
+
+/// Compile-time debug level for this Rust build (matches C `DEBUGLEVEL` default 0).
+pub const DEBUGLEVEL: i32 = 0;
+
+#[inline]
+pub fn debug_enabled(level: i32) -> bool {
+    DEBUGLEVEL >= 2 && level <= unsafe { g_debuglevel }
+}
+
+/// No-op when DEBUGLEVEL < 2; otherwise would log to stderr.
+#[macro_export]
+macro_rules! RAWLOG {
+    ($level:expr, $($arg:tt)*) => {{
+        if $crate::debug::debug_enabled($level) {
+            eprint!($($arg)*);
+        }
+    }};
+}
+
+/// No-op when DEBUGLEVEL < 2; otherwise would log file:line prefix + message.
+#[macro_export]
+macro_rules! DEBUGLOG {
+    ($level:expr, $($arg:tt)*) => {{
+        if $crate::debug::debug_enabled($level) {
+            eprint!("{}:{}: ", file!(), line!());
+            eprintln!($($arg)*);
+        }
+    }};
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn default_build_has_logging_disabled() {
+        assert_eq!(DEBUGLEVEL, 0);
+        assert!(!debug_enabled(i32::MIN));
+    }
+
+    #[test]
+    fn logging_macros_accept_format_arguments() {
+        RAWLOG!(2, "{}", 1);
+        DEBUGLOG!(2, "{}", 1);
+    }
+}