Remove fstest dependency on root package

[dmcgowan]

Related to #277

Summary

• Replace fstest's use of continuity.NewContext, continuity.BuildManifest, and the continuity.Resource interfaces with a local directory walker and plain resource struct
• Break the transitive dependency chain where consumers of CheckDirectoryEqual (used in ~37 test files across containerd and buildkit) pull in the full root package with its protobuf, manifest, and driver machinery
• Add test coverage for CheckDirectoryEqual, CheckDirectoryEqualWithApplier, and the new buildResources walker

Details

The fs/fstest package provides CheckDirectoryEqual for comparing two directory trees in tests. Previously this worked by building full continuity manifests (context + manifest + protobuf resources), which dragged in the entire root continuity package as a dependency. Any external repo importing fs/fstest for test helpers transitively required protobuf, the driver framework, digest libraries, and other machinery unrelated to simple directory comparison.

The new implementation replaces this with:

• walker.go — a filepath.Walk-based directory walker that produces a sorted []resource list with path, mode, uid/gid, size, SHA256 content hash, symlink target, and device major/minor
• walker_unix.go / walker_windows.go — platform-specific stat, hardlink detection, and device info extraction
• continuity_util.go — updated diff and comparison functions operating on the concrete resource type instead of continuity.Resource interfaces

The comparison behavior is preserved:

• Path, mode, uid, gid are compared for all resource types
• Regular files compare size, SHA256 digest, and hardlink paths
• Symlinks compare targets
• Devices compare major/minor numbers
• Directories and named pipes compare by metadata only
• XAttrs were collected but never compared before (// TODO in old code) and remain uncompared

Test plan

• New tests in compare_test.go cover: identical directories, content differences, extra/missing files, symlinks, symlink target differences, hardlinks, permission differences, applier-based comparison, and buildResources output validation
• go test ./fs/fstest/ passes
• Existing fs/ test suite (TestDirectoryCompare, copy tests using CheckDirectoryEqual) continues to pass