feat(compress): move match-state salt update into Rust

Move the row-matchfinder hash-salt field update behind a Rust in-place leaf.
C retains the private match-state pointer and entropy field access, while Rust
owns the scalar mutation and reuses the tested salt arithmetic.

Test Plan:
- cargo fmt --manifest-path rust/Cargo.toml -- --check
- ulimit -v 41943040 && CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- ulimit -v 41943040 && CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml zstd_compress::tests::bitmix_and_hash_salt_match_the_c_arithmetic -- --nocapture
- ulimit -v 41943040 && make -j1
- ulimit -v 41943040 && make -j1 -C tests test-fuzzer FUZZERTEST=-T3s FUZZER_FLAGS=--no-big-tests

lib/compress/zstd_compress.c

@@ -652,6 +652,7 @@ size_t ZSTD_compressStream2_c(ZSTD_CCtx* cctx,
 int ZSTD_rust_simpleCompress2Level(const void* cctx, size_t srcSize);
 int ZSTD_rust_simpleCompressStream2Level(const void* cctx, size_t srcSize);
 U32 ZSTD_rust_limitNextToUpdate(U32 curr, U32 nextToUpdate);
+void ZSTD_rust_advanceHashSaltInPlace(U64* hashSalt, U64 hashSaltEntropy);
 void ZSTD_rust_reduceIndex(U32* hashTable, U32 hashSize,
                            U32* chainTable, U32 chainSize,
                            U32* hashTable3, U32 hashSize3,
@@ -3756,10 +3757,6 @@ typedef enum {
 } ZSTD_resetTarget_e;
 
 /* Mixes in the hashSalt and hashSaltEntropy to create a new hashSalt */
-static void ZSTD_advanceHashSalt(ZSTD_MatchState_t* ms) {
-    ms->hashSalt = ZSTD_rust_advanceHashSalt(ms->hashSalt, (U64)ms->hashSaltEntropy);
-}
-
 typedef struct {
     ZSTD_MatchState_t* ms;
     ZSTD_cwksp* ws;
@@ -3829,7 +3826,8 @@ static void ZSTD_rust_resetMatchState_advanceHashSalt(void* opaque)
 {
     ZSTD_rust_resetMatchStateContext* const context =
             (ZSTD_rust_resetMatchStateContext*)opaque;
-    ZSTD_advanceHashSalt(context->ms);
+    ZSTD_rust_advanceHashSaltInPlace(
+            &context->ms->hashSalt, (U64)context->ms->hashSaltEntropy);
 }
 
 static void* ZSTD_rust_resetMatchState_reserve(

rust/src/zstd_compress.rs

@@ -5599,6 +5599,19 @@ pub extern "C" fn ZSTD_rust_advanceHashSalt(hash_salt: u64, hash_salt_entropy: u
     advance_hash_salt(hash_salt, hash_salt_entropy)
 }
 
+/// Advance a C-owned row-matchfinder salt in place while C retains the
+/// private match-state layout.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_advanceHashSaltInPlace(
+    hash_salt: *mut u64,
+    hash_salt_entropy: u64,
+) {
+    debug_assert!(!hash_salt.is_null());
+    unsafe {
+        *hash_salt = advance_hash_salt(*hash_salt, hash_salt_entropy);
+    }
+}
+
 #[inline]
 fn index_too_close_to_max(next_src_base_offset: usize) -> bool {
     next_src_base_offset > ZSTD_CURRENT_MAX - ZSTD_INDEXOVERFLOW_MARGIN
@@ -11243,6 +11256,9 @@ mod tests {
             ZSTD_rust_advanceHashSalt(0x0123_4567_89ab_cdef, 0xfedc_ba98_7654_3210),
             0xe5ee_f172_e5ff_3e57
         );
+        let mut hash_salt = 0x0123_4567_89ab_cdef;
+        unsafe { ZSTD_rust_advanceHashSaltInPlace(&mut hash_salt, 0xfedc_ba98_7654_3210) };
+        assert_eq!(hash_salt, 0xe5ee_f172_e5ff_3e57);
     }
 
     #[test]