oras-go: Arbitrary file write outside file.Store root via symlink-chain bypass in tar extraction (pushDir)

### Summary The `content/file.Store` in oras-go v2 unpacks OCI layer tarballs when a descriptor carries `io.deis.oras.content.unpack=true`. The extraction routine validates symlink targets purely lexically (`filepath.Join`) and, for regular files placed directly at the extraction root, skips the parent-symlink `Lstat` walk. A malicious tarball can plant a chain of symlinks whose lexical target stays inside the extraction root but whose kernel-resolved target is any absolute path, then write through it with a follow-up regular-file entry. The result is arbitrary file create/overwrite outside the store's working directory under the default `AllowPathTraversalOnWrite=false` configuration — a canonical tar-slip → RCE primitive. ### Details **Affected versions:** `<= v2.6.1` **Entry point:** `content/file/file.go` line 486, `(*Store).pushDir` — reached from `(*Store).Push` for any descriptor whose annotations include `io.deis.oras.content.unpack: "true"` (i.e. `file.AnnotationUnpack`) and an `org.opencontainers.image.title`. `oras.Copy` from a remote registry into a `file.New(dir)` store invokes this per layer. **Root cause 1 — lexical link validation.** `content/file/utils.go` lines 264–275, `ensureLinkPath`: ```go func ensureLinkPath(baseAbs, baseRel, link, target string) (string, error) { // resolve link path := target if !filepath.IsAbs(target) { path = filepath.Join(filepath.Dir(link), target) } // ensure path is under baseAbs or baseRel if _, err := resolveRelToBase(baseAbs,