feat(compress): move MT context size aggregation to Rust

ZSTDMT_sizeof_CCtx previously performed the complete size_t sum in C,
although every component depends on C-owned MT state and sizing helpers.
Keep the NULL guard, pool and dictionary queries, job-table multiplication,
and round-buffer extraction in C. Pass those eight computed components to a
Rust ABI helper that performs the same ordered wrapping additions.

This keeps MT context layout and allocator ownership on the C side while
moving only the scalar arithmetic across the existing C/Rust boundary.

Test Plan:
- `cargo clippy --manifest-path rust/Cargo.toml --no-default-features --features compression`
- Same clippy command with `--benches` and `--tests`, nightly fmt, then all three clippy commands again -- passed.
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression` -- 289 passed.
- `make -B -C lib -j2 lib` -- passed.
- `make -C tests test-rust-lib-smoke` -- passed.
- `make -B -C tests -j2 fuzzer` and `tests/fuzzer -s4560 -t56 -i57 -v` -- passed.
- `make -C tests -j2 test-zstream` -- 84 named, 6,268, and 8,862 randomized cases passed.
- `git diff --check` and `git diff --cached --check` -- passed.

The zstream build retains its pre-existing unterminated-string warning at
`tests/zstreamtest.c:1899`.

lib/compress/zstdmt_compress.c

@@ -151,6 +151,10 @@ void ZSTDMT_rust_findSynchronizationPoint(const void* inputSrc, size_t inputSize
                                            size_t* toLoad, int* flush);
 size_t ZSTDMT_rust_nextInputSizeHint(size_t targetSectionSize,
                                      size_t inBuffFilled);
+size_t ZSTDMT_rust_sizeofCCtx(size_t mtctxSize, size_t factorySize,
+                              size_t bufferPoolSize, size_t jobsSize,
+                              size_t cctxPoolSize, size_t seqPoolSize,
+                              size_t cdictSize, size_t roundBuffSize);
 
 typedef struct ZSTDMT_bufferPool_s {
     ZSTDMT_RustBufferPool* rustPool;
@@ -1029,15 +1033,26 @@ size_t ZSTDMT_freeCCtx(ZSTDMT_CCtx* mtctx)
 
 size_t ZSTDMT_sizeof_CCtx(ZSTDMT_CCtx* mtctx)
 {
+    size_t mtctxSize;
+    size_t factorySize;
+    size_t bufferPoolSize;
+    size_t jobsSize;
+    size_t cctxPoolSize;
+    size_t seqPoolSize;
+    size_t cdictSize;
+    size_t roundBuffSize;
     if (mtctx == NULL) return 0;   /* supports sizeof NULL */
-    return sizeof(*mtctx)
-            + POOL_sizeof(mtctx->factory)
-            + ZSTDMT_sizeof_bufferPool(mtctx->bufPool)
-            + (mtctx->jobIDMask+1) * sizeof(ZSTDMT_jobDescription)
-            + ZSTDMT_sizeof_CCtxPool(mtctx->cctxPool)
-            + ZSTDMT_sizeof_seqPool(mtctx->seqPool)
-            + ZSTD_sizeof_CDict(mtctx->cdictLocal)
-            + mtctx->roundBuff.capacity;
+    mtctxSize = sizeof(*mtctx);
+    factorySize = POOL_sizeof(mtctx->factory);
+    bufferPoolSize = ZSTDMT_sizeof_bufferPool(mtctx->bufPool);
+    jobsSize = (mtctx->jobIDMask+1) * sizeof(ZSTDMT_jobDescription);
+    cctxPoolSize = ZSTDMT_sizeof_CCtxPool(mtctx->cctxPool);
+    seqPoolSize = ZSTDMT_sizeof_seqPool(mtctx->seqPool);
+    cdictSize = ZSTD_sizeof_CDict(mtctx->cdictLocal);
+    roundBuffSize = mtctx->roundBuff.capacity;
+    return ZSTDMT_rust_sizeofCCtx(mtctxSize, factorySize, bufferPoolSize,
+                                  jobsSize, cctxPoolSize, seqPoolSize,
+                                  cdictSize, roundBuffSize);
 }