feat(compress): move dedicated dict hash-log transforms to Rust

Keep dedicated-dictionary strategy dispatch, parameter selection, and call-site
control flow in C while routing only the hashLog field transformations through
scalar Rust ABI helpers. The helpers use wrapping u32 arithmetic so overflow
and the existing subtract-then-clamp behavior remain identical to C.

Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression dedicated_dict_search_hash_log_adjustments_match_strategy_cases` -- passed
- `make -B -C lib -j2 lib` -- passed
- `make -C tests -j2 fuzzer` -- passed
- `./tests/fuzzer -s4560 -t47 -i48 -v` -- passed
- clippy normal, benches, and tests before and after formatting -- passed
- `cargo +nightly fmt --manifest-path rust/Cargo.toml` and `git diff --check` -- passed

lib/compress/zstd_compress.c

@@ -122,6 +122,8 @@ int ZSTD_rust_params_resolveEnableLdm(
 int ZSTD_rust_params_resolveExternalRepcodeSearch(int mode, int cLevel);
 int ZSTD_rust_params_cdictIndicesAreTagged(ZSTD_compressionParameters cParams);
 int ZSTD_rust_params_dedicatedDictSearchIsSupported(ZSTD_compressionParameters cParams);
+U32 ZSTD_rust_params_dedicatedDictSearch_getHashLog(U32 hashLog);
+U32 ZSTD_rust_params_dedicatedDictSearch_revertHashLog(U32 hashLog);
 
 /* CCtx parameter state is mirrored by rust/src/zstd_compress_params_api.rs.
  * Rust owns parameter bounds and clamping; C exposes only the
@@ -5663,7 +5665,7 @@ static ZSTD_compressionParameters ZSTD_dedicatedDictSearch_getCParams(int const
         case ZSTD_greedy:
         case ZSTD_lazy:
         case ZSTD_lazy2:
-            cParams.hashLog += ZSTD_LAZY_DDSS_BUCKET_LOG;
+            cParams.hashLog = ZSTD_rust_params_dedicatedDictSearch_getHashLog(cParams.hashLog);
             break;
         case ZSTD_btlazy2:
         case ZSTD_btopt:
@@ -5688,10 +5690,7 @@ static void ZSTD_dedicatedDictSearch_revertCParams(
         case ZSTD_greedy:
         case ZSTD_lazy:
         case ZSTD_lazy2:
-            cParams->hashLog -= ZSTD_LAZY_DDSS_BUCKET_LOG;
-            if (cParams->hashLog < ZSTD_HASHLOG_MIN) {
-                cParams->hashLog = ZSTD_HASHLOG_MIN;
-            }
+            cParams->hashLog = ZSTD_rust_params_dedicatedDictSearch_revertHashLog(cParams->hashLog);
             break;
         case ZSTD_btlazy2:
         case ZSTD_btopt:

rust/src/zstd_compress_params.rs

@@ -32,6 +32,7 @@ const ZSTD_WINDOWLOG_MAX: c_int = 30;
 const ZSTD_WINDOWLOG_MAX: c_int = 31;
 const ZSTD_HASHLOG_MIN: c_int = 6;
 const ZSTD_HASHLOG_MAX: c_int = 30;
+const ZSTD_LAZY_DDSS_BUCKET_LOG: u32 = 2;
 const ZSTD_CHAINLOG_MIN: c_int = ZSTD_HASHLOG_MIN;
 #[cfg(target_pointer_width = "32")]
 const ZSTD_CHAINLOG_MAX: c_int = 29;
@@ -439,6 +440,18 @@ fn dedicated_dict_search_is_supported(cparams: ZSTD_compressionParameters) -> bo
         && cparams.chainLog <= 24
 }
 
+#[inline]
+fn dedicated_dict_search_get_hash_log(hash_log: u32) -> u32 {
+    hash_log.wrapping_add(ZSTD_LAZY_DDSS_BUCKET_LOG)
+}
+
+#[inline]
+fn dedicated_dict_search_revert_hash_log(hash_log: u32) -> u32 {
+    hash_log
+        .wrapping_sub(ZSTD_LAZY_DDSS_BUCKET_LOG)
+        .max(ZSTD_HASHLOG_MIN as u32)
+}
+
 /// C ABI for `ZSTD_CDictIndicesAreTagged()`.
 #[no_mangle]
 pub extern "C" fn ZSTD_rust_params_cdictIndicesAreTagged(
@@ -455,6 +468,18 @@ pub extern "C" fn ZSTD_rust_params_dedicatedDictSearchIsSupported(
     c_int::from(dedicated_dict_search_is_supported(cparams))
 }
 
+/// C ABI for the hash-log adjustment used by `ZSTD_dedicatedDictSearch_getCParams()`.
+#[no_mangle]
+pub extern "C" fn ZSTD_rust_params_dedicatedDictSearch_getHashLog(hash_log: u32) -> u32 {
+    dedicated_dict_search_get_hash_log(hash_log)
+}
+
+/// C ABI for the hash-log adjustment used by `ZSTD_dedicatedDictSearch_revertCParams()`.
+#[no_mangle]
+pub extern "C" fn ZSTD_rust_params_dedicatedDictSearch_revertHashLog(hash_log: u32) -> u32 {
+    dedicated_dict_search_revert_hash_log(hash_log)
+}
+
 #[inline]
 fn dict_and_window_log(window_log: u32, src_size: u64, dict_size: u64) -> u32 {
     /* 1ULL << ZSTD_WINDOWLOG_MAX, which is smaller for 32-bit builds. */
@@ -1258,6 +1283,61 @@ mod tests {
         assert_eq!(supported(ZSTD_LAZY2, 25, 26), 0);
     }
 
+    #[test]
+    fn dedicated_dict_search_hash_log_adjustments_match_strategy_cases() {
+        let strategies = [
+            ZSTD_FAST,
+            ZSTD_DFAST,
+            ZSTD_GREEDY,
+            ZSTD_LAZY,
+            ZSTD_LAZY2,
+            ZSTD_BTLAZY2,
+            ZSTD_BTOPT,
+            ZSTD_BTULTRA,
+            ZSTD_BTULTRA2,
+        ];
+
+        for strategy in strategies {
+            let should_adjust = matches!(strategy, ZSTD_GREEDY | ZSTD_LAZY | ZSTD_LAZY2);
+            let original = 12;
+            let adjusted = if should_adjust {
+                ZSTD_rust_params_dedicatedDictSearch_getHashLog(original)
+            } else {
+                original
+            };
+            assert_eq!(adjusted, if should_adjust { 14 } else { original });
+
+            let reverted = if should_adjust {
+                ZSTD_rust_params_dedicatedDictSearch_revertHashLog(adjusted)
+            } else {
+                adjusted
+            };
+            assert_eq!(reverted, original);
+        }
+
+        assert_eq!(ZSTD_rust_params_dedicatedDictSearch_getHashLog(u32::MAX), 1);
+        assert_eq!(
+            ZSTD_rust_params_dedicatedDictSearch_revertHashLog(ZSTD_HASHLOG_MIN as u32),
+            ZSTD_HASHLOG_MIN as u32
+        );
+        assert_eq!(
+            ZSTD_rust_params_dedicatedDictSearch_revertHashLog(ZSTD_HASHLOG_MIN as u32 + 1),
+            ZSTD_HASHLOG_MIN as u32
+        );
+        assert_eq!(
+            ZSTD_rust_params_dedicatedDictSearch_revertHashLog(ZSTD_HASHLOG_MIN as u32 + 2),
+            ZSTD_HASHLOG_MIN as u32
+        );
+        assert_eq!(
+            ZSTD_rust_params_dedicatedDictSearch_revertHashLog(0),
+            u32::MAX - 1
+        );
+        assert_eq!(
+            ZSTD_rust_params_dedicatedDictSearch_revertHashLog(1),
+            u32::MAX
+        );
+    }
+
     #[test]
     fn external_sequence_validation_preserves_its_int_mode() {
         for mode in [c_int::MIN, -1, 0, 1, c_int::MAX] {