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.

rust/src/zstdmt_compress.rs

@@ -686,6 +686,16 @@ pub unsafe extern "C" fn ZSTDMT_rust_buffer_pool_sizeof(pool: *const RustBufferP
         .saturating_add(total_buffer_size)
 }
 
+#[inline]
+fn sizeof_buffer_pool(wrapper_size: usize, rust_pool_size: usize) -> usize {
+    wrapper_size.wrapping_add(rust_pool_size)
+}
+
+#[no_mangle]
+pub extern "C" fn ZSTDMT_rust_sizeofBufferPool(wrapperSize: usize, rustPoolSize: usize) -> usize {
+    sizeof_buffer_pool(wrapperSize, rustPoolSize)
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn ZSTDMT_rust_buffer_pool_set_size(
     pool: *mut RustBufferPool,
@@ -1608,4 +1618,22 @@ mod tests {
         assert_eq!(sizeof_cctx_pool(usize::MAX, 1), 0);
         assert_eq!(sizeof_cctx_pool(usize::MAX - 7, 11), 3);
     }
+
+    #[test]
+    fn buffer_pool_size_addition_handles_zero_components() {
+        assert_eq!(sizeof_buffer_pool(0, 0), 0);
+        assert_eq!(sizeof_buffer_pool(37, 0), 37);
+        assert_eq!(sizeof_buffer_pool(0, 53), 53);
+    }
+
+    #[test]
+    fn buffer_pool_size_addition_preserves_ordinary_sum() {
+        assert_eq!(sizeof_buffer_pool(128, 4096), 4224);
+    }
+
+    #[test]
+    fn buffer_pool_size_addition_wraps_like_c_size_t() {
+        assert_eq!(sizeof_buffer_pool(usize::MAX, 1), 0);
+        assert_eq!(sizeof_buffer_pool(usize::MAX - 7, 11), 3);
+    }
 }