feat(compress): move C parameter overrides to Rust

The compression-parameter override helper previously lived in C and selectively
copied each nonzero field from a caller-provided override structure. Move that
pure field-wise operation into the Rust compression-parameter module, while
keeping the existing repr(C) structure and pointer-based ABI at both C call
sites. Zero-valued overrides continue to leave the current parameter untouched.

Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression` -- passed (252 tests)
- `make -B -C lib -j2 lib` -- passed
- `make -C tests -j2 fuzzer` and `tests/fuzzer -s4560 -t47 -i48 -v` -- passed
- required clippy passes, nightly fmt, and `git diff --check` -- passed

rust/src/zstd_compress_params.rs

@@ -905,6 +905,44 @@ fn get_block_size(max_block_size: usize, window_log: u32) -> usize {
     max_block_size.min(1usize << window_log)
 }
 
+#[inline]
+fn override_cparams(
+    cparams: &mut ZSTD_compressionParameters,
+    overrides: &ZSTD_compressionParameters,
+) {
+    if overrides.windowLog != 0 {
+        cparams.windowLog = overrides.windowLog;
+    }
+    if overrides.hashLog != 0 {
+        cparams.hashLog = overrides.hashLog;
+    }
+    if overrides.chainLog != 0 {
+        cparams.chainLog = overrides.chainLog;
+    }
+    if overrides.searchLog != 0 {
+        cparams.searchLog = overrides.searchLog;
+    }
+    if overrides.minMatch != 0 {
+        cparams.minMatch = overrides.minMatch;
+    }
+    if overrides.targetLength != 0 {
+        cparams.targetLength = overrides.targetLength;
+    }
+    if overrides.strategy != 0 {
+        cparams.strategy = overrides.strategy;
+    }
+}
+
+/// Pure leaf for the private `ZSTD_overrideCParams()` helper.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_params_overrideCParams(
+    cparams: *mut ZSTD_compressionParameters,
+    overrides: *const ZSTD_compressionParameters,
+) {
+    let (cparams, overrides) = unsafe { (&mut *cparams, &*overrides) };
+    override_cparams(cparams, overrides);
+}
+
 /// Pure leaf for the deprecated `ZSTD_getBlockSize()` path.
 #[no_mangle]
 pub extern "C" fn ZSTD_rust_params_getBlockSize(maxBlockSize: usize, windowLog: u32) -> usize {
@@ -1337,4 +1375,41 @@ mod tests {
         assert_eq!(ZSTD_rust_params_getBlockSize(128 * 1024, 16), 64 * 1024);
         assert_eq!(ZSTD_rust_params_getBlockSize(128 * 1024, 17), 128 * 1024);
     }
+
+    #[test]
+    fn cparam_overrides_replace_only_nonzero_fields() {
+        let mut cparams = ZSTD_compressionParameters {
+            windowLog: 10,
+            chainLog: 11,
+            hashLog: 12,
+            searchLog: 13,
+            minMatch: 14,
+            targetLength: 15,
+            strategy: 16,
+        };
+        let overrides = ZSTD_compressionParameters {
+            windowLog: 20,
+            chainLog: 0,
+            hashLog: 22,
+            searchLog: 0,
+            minMatch: 24,
+            targetLength: 0,
+            strategy: 26,
+        };
+
+        unsafe { ZSTD_rust_params_overrideCParams(&mut cparams, &overrides) };
+
+        assert_eq!(
+            cparams,
+            ZSTD_compressionParameters {
+                windowLog: 20,
+                chainLog: 11,
+                hashLog: 22,
+                searchLog: 13,
+                minMatch: 24,
+                targetLength: 15,
+                strategy: 26,
+            }
+        );
+    }
 }