feat(mt): move frame checksum finalization to Rust

The MT flush path already delegated checksum serialization to Rust but still
computed XXH64_digest in C. Pass the live C-owned XXH64 state through the
checked projection and let Rust finalize the digest before the existing
little-endian write. C retains serial state storage and job/flush mutation.

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
- Focused cargo test compiles but remains link-blocked by existing C-owned decompression-view symbols.

lib/compress/zstdmt_compress.c

@@ -466,7 +466,7 @@ typedef struct {
     void* dst;
     size_t dstCapacity;
     size_t cSize;
-    U32 checksum;
+    const XXH64_state_t* checksumState;
 } ZSTDMT_RustFrameChecksumProjection;
 typedef char ZSTDMT_frame_checksum_projection_layout[
         (offsetof(ZSTDMT_RustFrameChecksumProjection, dst) == 0
@@ -474,9 +474,8 @@ typedef char ZSTDMT_frame_checksum_projection_layout[
                 == sizeof(void*)
          && offsetof(ZSTDMT_RustFrameChecksumProjection, cSize)
                 == 2 * sizeof(void*)
-         && offsetof(ZSTDMT_RustFrameChecksumProjection, checksum)
+         && offsetof(ZSTDMT_RustFrameChecksumProjection, checksumState)
                 == 3 * sizeof(void*)
-         && sizeof(U32) == 4
          && sizeof(ZSTDMT_RustFrameChecksumProjection)
                 == 4 * sizeof(void*))
         ? 1 : -1];
@@ -3440,12 +3439,11 @@ static void ZSTDMT_addFrameChecksum(void* opaque, unsigned jobID,
 {
     ZSTDMT_CCtx* const mtctx = (ZSTDMT_CCtx*)opaque;
     ZSTDMT_jobDescription* const job = &mtctx->jobs[jobID];
-    U32 const checksum = (U32)XXH64_digest(&mtctx->serial.xxhState);
     ZSTDMT_RustFrameChecksumProjection const checksumProjection = {
         job->dstBuff.start,
         job->dstBuff.capacity,
         job->cSize,
-        checksum
+        &mtctx->serial.xxhState
     };
     size_t const cSize = ZSTDMT_rust_writeFrameChecksum(&checksumProjection);