feat(compress): move sequence validation identity to Rust

ZSTD_resolveExternalSequenceValidation() is a pure int identity used while
resolving CCtx parameters. Route its existing C wrapper through a Rust
extern "C" entry point in the parameter module, preserving the int-to-int
ABI and every call-site result. Add boundary coverage for signed int modes.

Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression` -- passed (253 tests)
- `make -B -C lib -j2 lib` -- passed
- required clippy passes for the compression feature, benches, and tests -- passed
- `cargo +nightly fmt --manifest-path rust/Cargo.toml` and `git diff --check` -- passed

rust/src/zstd_compress_params.rs

@@ -415,6 +415,17 @@ pub extern "C" fn ZSTD_rust_params_resolveExternalRepcodeSearch(
     resolve_external_repcode_search(mode, compression_level)
 }
 
+#[inline]
+fn resolve_external_sequence_validation(mode: c_int) -> c_int {
+    mode
+}
+
+/// C ABI for `ZSTD_resolveExternalSequenceValidation()`.
+#[no_mangle]
+pub extern "C" fn ZSTD_rust_params_resolveExternalSequenceValidation(mode: c_int) -> c_int {
+    resolve_external_sequence_validation(mode)
+}
+
 #[inline]
 fn cdict_indices_are_tagged(cparams: ZSTD_compressionParameters) -> bool {
     cparams.strategy == ZSTD_FAST || cparams.strategy == ZSTD_DFAST
@@ -1213,6 +1224,16 @@ mod tests {
         );
     }
 
+    #[test]
+    fn external_sequence_validation_preserves_its_int_mode() {
+        for mode in [c_int::MIN, -1, 0, 1, c_int::MAX] {
+            assert_eq!(
+                ZSTD_rust_params_resolveExternalSequenceValidation(mode),
+                mode
+            );
+        }
+    }
+
     #[test]
     fn level_tables_match_representative_clevels_entries() {
         let large = select_cparams(3, ZSTD_CONTENTSIZE_UNKNOWN, 0, ZSTD_RUST_CPM_UNKNOWN);