feat(compress): move CCtx size aggregation to Rust

Keep ZSTD_sizeof_CCtx's NULL handling, workspace accounting, local-dictionary
sizing, and multithreaded context sizing in C, where the private layouts and
ownership remain visible. Pass only the four resulting size_t values to a Rust
ABI helper, which performs the final ordered additions with C-compatible
wrapping semantics. Add focused coverage for zero, ordinary, and overflowing
component sums.

Test Plan:
- `cargo clippy --manifest-path rust/Cargo.toml --no-default-features --features compression` -- passed before and after formatting
- Same clippy command with `--benches` and `--tests` -- passed before and after formatting
- `cargo +nightly fmt --manifest-path rust/Cargo.toml` -- passed
- `cargo test --manifest-path rust/Cargo.toml --no-default-features --features compression sizeof_cctx` -- passed (6 tests)
- `make -B -C lib -j2 lib` -- passed
- `make -C tests test-rust-lib-smoke` -- passed
- `tests/fuzzer -s4560 -t56 -i57 -v` -- passed
- `make -C tests -j2 test-zstream` -- passed (84 named, 6252 and 9519 randomized)
- `git diff --check` and `git diff --cached --check` -- passed

lib/compress/zstd_compress.c

@@ -90,6 +90,8 @@ size_t ZSTD_rust_CStreamInSize(void);
 size_t ZSTD_rust_CStreamOutSize(void);
 size_t ZSTD_rust_sizeofLocalDict(int dictBufferPresent, size_t dictSize,
                                  size_t cdictSize);
+size_t ZSTD_rust_sizeofCCtx(size_t objectSize, size_t workspaceSize,
+                            size_t localDictSize, size_t mtctxSize);
 ZSTD_inBuffer ZSTD_rust_inBufferForEndFlush(int inBufferMode,
                                             const void* expectedSrc,
                                             size_t expectedSize,
@@ -582,11 +584,15 @@ static size_t ZSTD_sizeof_mtctx(const ZSTD_CCtx* cctx)
 size_t ZSTD_sizeof_CCtx(const ZSTD_CCtx* cctx)
 {
     if (cctx==NULL) return 0;   /* support sizeof on NULL */
-    /* cctx may be in the workspace */
-    return (cctx->workspace.workspace == cctx ? 0 : sizeof(*cctx))
-           + ZSTD_cwksp_sizeof(&cctx->workspace)
-           + ZSTD_sizeof_localDict(cctx->localDict)
-           + ZSTD_sizeof_mtctx(cctx);
+    {
+        /* cctx may be in the workspace */
+        size_t const objectSize = cctx->workspace.workspace == cctx ? 0 : sizeof(*cctx);
+        size_t const workspaceSize = ZSTD_cwksp_sizeof(&cctx->workspace);
+        size_t const localDictSize = ZSTD_sizeof_localDict(cctx->localDict);
+        size_t const mtctxSize = ZSTD_sizeof_mtctx(cctx);
+        return ZSTD_rust_sizeofCCtx(objectSize, workspaceSize, localDictSize,
+                                    mtctxSize);
+    }
 }
 
 size_t ZSTD_sizeof_CStream(const ZSTD_CStream* zcs)