feat(compress): move repcode invalidation to Rust

Keep ZSTD_invalidateRepCodes responsible for extracting the private previous
block state and checking that the match window has no external dictionary.
Delegate only the fixed three-entry repcode reset through a narrow pointer ABI,
with a C assertion preserving the Rust side's ZSTD_REP_NUM assumption.

Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression zstd_compress::tests::invalidate_rep_codes_clears_all_entries` -- passed
- `make -B -C lib -j2 lib` -- passed
- `make -C tests -j2 fuzzer` and `./tests/fuzzer -s4560 -t47 -i48 -v` -- passed
- Required clippy, nightly fmt, and diff checks -- passed

rust/src/zstd_compress.rs

@@ -72,6 +72,7 @@ unsafe extern "C" {
 
 const ZSTD_FAST: c_int = 1;
 const ZSTD_DFAST: c_int = 2;
+const ZSTD_REP_NUM: usize = 3;
 #[cfg(test)]
 const ZSTD_BM_BUFFERED: c_int = 0;
 const ZSTD_BM_STABLE: c_int = 1;
@@ -365,6 +366,14 @@ pub unsafe extern "C" fn ZSTD_rust_copyCDictTableIntoCCtx(
     }
 }
 
+/// Clear the previous block's repcodes before the next regular compression.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_invalidateRepCodes(rep: *mut u32) {
+    debug_assert!(!rep.is_null());
+    let rep = unsafe { std::slice::from_raw_parts_mut(rep, ZSTD_REP_NUM) };
+    rep.fill(0);
+}
+
 #[inline]
 fn zeroed_state() -> ZSTD_compressedBlockState_t {
     /* The state contains only integer arrays and enum fields. */
@@ -1118,6 +1127,15 @@ mod tests {
         );
     }
 
+    #[test]
+    fn invalidate_rep_codes_clears_all_entries() {
+        let mut rep = [11u32, 22, 33];
+
+        unsafe { ZSTD_rust_invalidateRepCodes(rep.as_mut_ptr()) };
+
+        assert_eq!(rep, [0; ZSTD_REP_NUM]);
+    }
+
     #[test]
     fn public_one_shot_abi_is_c_compatible() {
         let entry: unsafe extern "C" fn(*mut c_void, usize, *const c_void, usize, c_int) -> usize =