feat(compress): move MT buffer pool size addition to Rust

Keep buffer-pool ownership, allocation, synchronization, and the Rust
pool's saturating accounting in their existing implementations.  The C
wrapper still handles NULL, obtains sizeof(*bufPool), and queries the Rust
pool size; Rust now performs only the final size_t addition with wrapping
semantics.  The sequence-pool sizing alias continues to call the same C
helper and therefore retains its behavior.

Test Plan:
- `cargo clippy --manifest-path rust/Cargo.toml --no-default-features
  --features compression` (before and after formatting) -- passed
- The same clippy command with `--benches` and `--tests` -- passed
- `cargo +nightly fmt --manifest-path rust/Cargo.toml` -- passed
- `cargo test --manifest-path rust/Cargo.toml --no-default-features
  --features compression zstdmt_compress` -- 29 passed
- `make -B -C lib -j2 lib` -- passed
- `make -C tests test-rust-lib-smoke` -- passed
- `tests/fuzzer -s4560 -t56 -i57 -v` -- 57 passed
- `make -C tests -j2 test-zstream` -- 84 named, 7,661, and 8,090
  randomized tests passed
- `git diff --check` and `git diff --cached --check` -- passed

The zstream build still emits the pre-existing unterminated-string warning
at tests/zstreamtest.c:1899.

lib/compress/zstdmt_compress.c

@@ -106,6 +106,7 @@ ZSTDMT_RustBufferPool* ZSTDMT_rust_buffer_pool_create(unsigned maxNbBuffers,
                                                        ZSTD_customMem cMem);
 void ZSTDMT_rust_buffer_pool_free(ZSTDMT_RustBufferPool* pool);
 size_t ZSTDMT_rust_buffer_pool_sizeof(const ZSTDMT_RustBufferPool* pool);
+size_t ZSTDMT_rust_sizeofBufferPool(size_t wrapperSize, size_t rustPoolSize);
 void ZSTDMT_rust_buffer_pool_set_size(ZSTDMT_RustBufferPool* pool, size_t bSize);
 ZSTDMT_RustBufferPool* ZSTDMT_rust_buffer_pool_expand(ZSTDMT_RustBufferPool* pool,
                                                        unsigned maxNbBuffers);
@@ -186,7 +187,8 @@ static ZSTDMT_bufferPool* ZSTDMT_createBufferPool(unsigned maxNbBuffers, ZSTD_cu
 static size_t ZSTDMT_sizeof_bufferPool(ZSTDMT_bufferPool* bufPool)
 {
     if (bufPool == NULL) return 0;
-    return sizeof(*bufPool) + ZSTDMT_rust_buffer_pool_sizeof(bufPool->rustPool);
+    return ZSTDMT_rust_sizeofBufferPool(
+        sizeof(*bufPool), ZSTDMT_rust_buffer_pool_sizeof(bufPool->rustPool));
 }
 
 /* ZSTDMT_setBufferSize() :