feat(compress): move MT rolling hash prime power to Rust

The MT rsync initialization previously computed the rolling-hash prime
power through the C-only ZSTD_rollingHash_primePower helper. Move only that
scalar exponentiation behind the existing Rust MT ABI boundary. The C caller
still owns the MT context assignment, RSYNC_LENGTH constant, and surrounding
initialization.

The Rust helper mirrors ZSTD_ipow with wrapping u64 multiplication and applies
u32 wrapping subtraction before widening the exponent, preserving the C
length == 0 behavior. Rolling-hash scanning, rotation, and all other MT state
remain unchanged.

Test Plan:
- Focused rolling_hash_prime_power Rust tests -- passed (2 tests).
- Required clippy/fmt sequence in normal, bench, and test modes -- passed.
- make -B -C lib -j2 lib -- passed.
- make -C tests test-rust-lib-smoke -- passed.
- tests/fuzzer -s4560 -t56 -i57 -v -- passed (57 tests).
- make -C tests -j2 test-zstream -- passed (84 named, 5,305 + 7,351 randomized).
- git diff --check and git diff --cached --check -- passed.

lib/compress/zstdmt_compress.c

@@ -150,6 +150,7 @@ void ZSTDMT_rust_findSynchronizationPoint(const void* inputSrc, size_t inputSize
                                            const void* inBuffStart, size_t inBuffFilled,
                                            int rsyncable, U64 primePower, U64 hitMask,
                                            size_t* toLoad, int* flush);
+U64 ZSTDMT_rust_rollingHashPrimePower(U32 length);
 size_t ZSTDMT_rust_nextInputSizeHint(size_t targetSectionSize,
                                      size_t inBuffFilled);
 size_t ZSTDMT_rust_sizeofCCtx(size_t mtctxSize, size_t factorySize,
@@ -1253,7 +1254,7 @@ size_t ZSTDMT_initCStream_internal(
         DEBUGLOG(4, "rsyncLog = %u", rsyncBits);
         mtctx->rsync.hash = 0;
         mtctx->rsync.hitMask = (1ULL << rsyncBits) - 1;
-        mtctx->rsync.primePower = ZSTD_rollingHash_primePower(RSYNC_LENGTH);
+        mtctx->rsync.primePower = ZSTDMT_rust_rollingHashPrimePower(RSYNC_LENGTH);
     }
     if (mtctx->targetSectionSize < mtctx->targetPrefixSize) mtctx->targetSectionSize = mtctx->targetPrefixSize;  /* job size must be >= overlap size */
     DEBUGLOG(4, "Job Size : %u KB (note : set to %u)", (U32)(mtctx->targetSectionSize>>10), (U32)params.jobSize);