WIP: Add OCI image signing, sealing, and signature-verified mount

Implement end-to-end support for cryptographically signing composefs
OCI images using PKCS#7/fsverity detached signatures, stored as OCI
referrer artifacts following the 'composefs erofs-alongside' spec.

Core signing infrastructure (composefs crate):
- Add fsverity algorithm constants and ComposeFsAlgorithm type
- Add formatted_digest module for kernel-compatible fsverity digest
  construction (the 12-byte header + raw hash used by the kernel's
  FS_IOC_ENABLE_VERITY ioctl)
- Add kernel keyring support via composefs-ioctls keyring module
  (inject X.509 certs into .fs-verity keyring for kernel-level
  signature enforcement)

OCI signing library (composefs-oci crate):
- signing.rs: FsVeritySigningKey (sign) and FsVeritySignatureVerifier
  (verify) using openssl PKCS#7 with DETACHED|BINARY|NOATTR flags,
  compatible with Linux kernel fsverity builtin signature verification
- signature.rs: OCI artifact manifest builder/parser for the
  'application/vnd.composefs.erofs-alongside.v1' artifact type,
  storing per-layer and merged EROFS images alongside their PKCS#7
  signatures as typed layers with composefs.* annotations
- image.rs: compute_per_layer_digests() and compute_merged_digest()
  for deterministic EROFS image generation from OCI layer stacks
- oci_image.rs: seal_image() to compute and embed the composefs
  fsverity digest into the OCI config, export/import to OCI layout
  directories (migrated to ocidir crate for atomic I/O), referrer
  index management

CLI commands (cfsctl):
- 'oci seal <image>' — compute composefs EROFS, embed fsverity digest
- 'oci sign <image> --cert --key' — create signature artifact
- 'oci verify <image> [--cert]' — verify signatures (digest-only
  without --cert, full PKCS#7 with --cert)
- 'oci mount <name> <mountpoint> [--require-signature --trust-cert]'
  — verify signatures before kernel mount
- 'oci pull ... --require-signature --trust-cert' — verify after pull
- 'oci push <image> <dest> [--signatures]' — export to OCI layout
- 'oci export-signatures <image> <dest>' — export just artifacts
- 'oci inspect' — show referrer info in JSON output
- 'keyring add-cert <pem>' — inject cert into kernel keyring

The mount and pull --require-signature paths share a common
verify_image_signatures() helper that recomputes expected EROFS
digests and verifies each PKCS#7 signature blob against the trusted
certificate.

The mount command now also resolves tag names (via OciImage::open_ref)
instead of requiring raw config digests, consistent with seal/sign/
verify.

Integration tests:
- signing.rs: 17 unprivileged tests covering sign, verify, wrong cert,
  export, seal+sign roundtrip, artifact structure, --require-signature
  on pull and mount
- privileged.rs: 7 tests for real fsverity enforcement, kernel keyring
  injection, kernel signature acceptance/rejection
- podman.rs: 3 tests building real container images via podman
- cli.rs: updated for richer OCI test layout (4 entries) and new
  oci push/roundtrip tests
- test-oci-sign-verify.sh: standalone shell-based integration tests

Assisted-by: OpenCode (Claude claude-opus-4-6)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

contrib/packaging/install-test-deps.sh

@@ -15,7 +15,7 @@ case "${ID}" in
         ;;
     debian|ubuntu)
         pkg_install \
-            openssl e2fsprogs bubblewrap openssh-server
+            openssl e2fsprogs bubblewrap openssh-server fsverity-utils
 
         # OSTree symlink targets — /root, /home, /srv, etc. are symlinks
         # into /var on OSTree systems, so the target directories must exist.

crates/cfsctl/Cargo.toml

@@ -16,7 +16,7 @@ path = "src/lib.rs"
 [features]
 default = ['pre-6.15', 'oci']
 http = ['composefs-http']
-oci = ['composefs-oci']
+oci = ['composefs-oci', 'oci-spec']
 rhel9 = ['composefs/rhel9']
 'pre-6.15' = ['composefs/pre-6.15']
 
@@ -31,6 +31,7 @@ composefs-oci = { workspace = true, optional = true, features = ["boot"] }
 composefs-http = { workspace = true, optional = true }
 env_logger = { version = "0.11.0", default-features = false }
 hex = { version = "0.4.0", default-features = false }
+oci-spec = { version = "0.8.0", default-features = false, optional = true }
 rustix = { version = "1.0.0", default-features = false, features = ["fs", "process"] }
 serde = { version = "1.0", default-features = false, features = ["derive"] }
 serde_json = { version = "1.0", default-features = false, features = ["std"] }