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

Keep ZSTDMT_sizeof_CCtxPool's NULL handling, wrapper sizeof, and Rust pool
size query in C while delegating only the final size_t aggregation to a
Rust ABI helper. The helper uses wrapping_add so its result matches C's
unsigned size_t arithmetic without changing the pool's existing saturating
internal accounting, layout, synchronization, or allocation paths.

Test Plan:
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression zstdmt_compress` -- passed (26 tests)
- Required clippy sequence before and after `cargo +nightly fmt` -- 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, 6,871 and 9,464 randomized tests)
- `git diff --check` and `git diff --cached --check` -- passed
- Existing warning at `tests/zstreamtest.c:1899` remains unchanged

rust/src/zstdmt_compress.rs

@@ -939,6 +939,16 @@ pub unsafe extern "C" fn ZSTDMT_rust_cctx_pool_sizeof(pool: *const RustCCtxPool)
         .saturating_add(total_cctx_size)
 }
 
+#[inline]
+fn sizeof_cctx_pool(wrapper_size: usize, rust_pool_size: usize) -> usize {
+    wrapper_size.wrapping_add(rust_pool_size)
+}
+
+#[no_mangle]
+pub extern "C" fn ZSTDMT_rust_sizeofCCtxPool(wrapperSize: usize, rustPoolSize: usize) -> usize {
+    sizeof_cctx_pool(wrapperSize, rustPoolSize)
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn ZSTDMT_rust_cctx_pool_expand(
     pool: *mut RustCCtxPool,
@@ -1580,4 +1590,22 @@ mod tests {
             1usize << 25
         );
     }
+
+    #[test]
+    fn cctx_pool_size_addition_handles_zero_components() {
+        assert_eq!(sizeof_cctx_pool(0, 0), 0);
+        assert_eq!(sizeof_cctx_pool(37, 0), 37);
+        assert_eq!(sizeof_cctx_pool(0, 53), 53);
+    }
+
+    #[test]
+    fn cctx_pool_size_addition_preserves_ordinary_sum() {
+        assert_eq!(sizeof_cctx_pool(128, 4096), 4224);
+    }
+
+    #[test]
+    fn cctx_pool_size_addition_wraps_like_c_size_t() {
+        assert_eq!(sizeof_cctx_pool(usize::MAX, 1), 0);
+        assert_eq!(sizeof_cctx_pool(usize::MAX - 7, 11), 3);
+    }
 }