feat(mt): move serial checksum update to Rust

The MT serial-turn policy already ran in Rust but still crossed back into C
for the XXH64 update. Pass the live C-owned checksum state through the explicit
ABI and let Rust perform the update after the existing LDM and turn gate. C
retains the private serial state, synchronization, and raw sequence storage.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo check --tests (rust)
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --tests -- -A clippy::manual-bits -D warnings (rust)
- ulimit -v 41943040; make -j1
- ulimit -v 41943040; make -j1 -C tests test
- Added a Rust checksum-vector test; standalone cargo test remains link-blocked by existing C-owned decompression-view symbols.

lib/compress/zstdmt_compress.c

@@ -713,8 +713,6 @@ typedef void (*ZSTDMT_serialLdmGenerateSequencesFn)(
         void* opaque, ZSTDMT_RustRawSeqStore* seqStore,
         const void* src, size_t srcSize);
 typedef void (*ZSTDMT_serialLdmPublishWindowFn)(void* opaque);
-typedef void (*ZSTDMT_serialUpdateChecksumFn)(
-        void* opaque, const void* src, size_t srcSize);
 typedef void (*ZSTDMT_serialAdvanceFn)(void* opaque);
 typedef void (*ZSTDMT_serialStateLockFn)(void* opaque);
 typedef void (*ZSTDMT_serialStateWaitFn)(void* opaque);
@@ -752,11 +750,11 @@ void ZSTDMT_rust_serialStateAdvance(const ZSTDMT_RustSerialAdvanceState* state);
 void ZSTDMT_rust_serialStateGenSequences(
         ZSTDMT_RustRawSeqStore* seqStore, const void* src, size_t srcSize,
         unsigned jobID, int ldmEnabled, int checksumEnabled, void* opaque,
+        XXH64_state_t* checksumState,
         ZSTDMT_serialWaitForTurnFn waitForTurn,
         ZSTDMT_serialLdmWindowUpdateFn updateLdmWindow,
         ZSTDMT_serialLdmGenerateSequencesFn generateLdmSequences,
         ZSTDMT_serialLdmPublishWindowFn publishLdmWindow,
-        ZSTDMT_serialUpdateChecksumFn updateChecksum,
         ZSTDMT_serialAdvanceFn advance);
 typedef struct {
     unsigned skip;
@@ -1892,13 +1890,6 @@ static void ZSTDMT_serialState_publishLdmWindow(void* opaque)
     ZSTD_pthread_mutex_unlock(&serialState->ldmWindowMutex);
 }
 
-static void ZSTDMT_serialState_updateChecksum(
-        void* opaque, const void* src, size_t srcSize)
-{
-    SerialState* const serialState = (SerialState*)opaque;
-    XXH64_update(&serialState->xxhState, src, srcSize);
-}
-
 static void ZSTDMT_serialState_advance(void* opaque)
 {
     SerialState* const serialState = (SerialState*)opaque;
@@ -2140,11 +2131,11 @@ static void ZSTDMT_compressionJobGenerateSequences(void* opaque)
             job->serial->params.ldmParams.enableLdm == ZSTD_ps_enable,
             job->serial->params.fParams.checksumFlag,
             job->serial,
+            &job->serial->xxhState,
             ZSTDMT_serialState_waitForTurn,
             ZSTDMT_serialState_updateLdmWindow,
             ZSTDMT_serialState_generateLdmSequences,
             ZSTDMT_serialState_publishLdmWindow,
-            ZSTDMT_serialState_updateChecksum,
             ZSTDMT_serialState_advance);
 }