Phase 1: imagepull package — pull, flatten, ext4

New internal/imagepull/ subpackage. Three concerns, each
independently testable:

Pull (imagepull.go):
 - github.com/google/go-containerregistry's remote.Image with the
   linux/amd64 platform pinned. Anonymous pulls only for v1.
 - Layer blobs cached on disk via cache.NewFilesystemCache under
   <cacheDir>/blobs/sha256/<hex> — OCI-standard layout so
   skopeo/crane could co-exist later.
 - Eagerly touches every layer once so network errors surface at
   Pull time, not deep in Flatten.

Flatten (flatten.go):
 - Replays layers oldest-first into destDir.
 - Whiteout-aware: .wh.<name> deletes the named entry,
   .wh..wh..opq wipes the parent directory's contents from prior
   layers.
 - Path-traversal hardening mirrored from kernelcat extractTar:
   reject .., absolute paths, and symlinks/hardlinks whose
   resolved target escapes destDir.
 - Handles tar.TypeReg, TypeDir, TypeSymlink, TypeLink. Skips
   device/fifo nodes silently (need privilege; udev/devtmpfs
   handles them in the guest).

BuildExt4 (ext4.go):
 - Truncates outFile to sizeBytes, then runs `mkfs.ext4 -F -d
   <srcDir> -E root_owner=0:0`. No mount, no sudo, no loopback.
 - 64 MiB floor; callers handle real sizing with content-aware
   headroom.
 - File ownership in the resulting ext4 reflects srcDir's on-disk
   ownership — runner's uid/gid since extraction was unprivileged.
   Documented in package doc as a Phase A v1 limitation; Phase B
   will add a debugfs- or tar2ext4-based ownership fixup.

paths.Layout gains OCICacheDir at $XDG_CACHE_HOME/banger/oci/,
ensured at startup alongside the other dirs.

Tests use go-containerregistry's in-process registry to push and
pull synthetic multi-layer images. Cover: layer caching round-trip,
whiteout + opaque-marker handling, path-traversal rejection, unsafe
symlink rejection, real mkfs.ext4 round-trip (skipped if mkfs.ext4
absent), and tiny-size rejection.

go-containerregistry v0.21.5 added as a direct dep, plus its
transitive closure (containerd/stargz, opencontainers/go-digest,
docker/cli config helpers, etc).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

go.mod

@@ -4,12 +4,14 @@ go 1.25.0
 
 require (
 	github.com/firecracker-microvm/firecracker-go-sdk v1.0.0
+	github.com/google/go-containerregistry v0.21.5
+	github.com/klauspost/compress v1.18.5
 	github.com/miekg/dns v1.1.72
 	github.com/pelletier/go-toml v1.9.5
 	github.com/sirupsen/logrus v1.9.4
-	github.com/spf13/cobra v1.8.1
-	golang.org/x/crypto v0.46.0
-	golang.org/x/sys v0.39.0
+	github.com/spf13/cobra v1.10.2
+	golang.org/x/crypto v0.50.0
+	golang.org/x/sys v0.43.0
 	modernc.org/sqlite v1.38.2
 )
 
@@ -18,8 +20,11 @@ require (
 	github.com/PuerkitoBio/urlesc v0.0.0-20170810143723-de5bf2ad4578 // indirect
 	github.com/asaskevich/govalidator v0.0.0-20210307081110-f21760c49a8d // indirect
 	github.com/containerd/fifo v1.0.0 // indirect
+	github.com/containerd/stargz-snapshotter/estargz v0.18.2 // indirect
 	github.com/containernetworking/cni v1.0.1 // indirect
 	github.com/containernetworking/plugins v1.0.1 // indirect
+	github.com/docker/cli v29.4.0+incompatible // indirect
+	github.com/docker/docker-credential-helpers v0.9.3 // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/go-openapi/analysis v0.21.2 // indirect
 	github.com/go-openapi/errors v0.20.2 // indirect
@@ -37,27 +42,30 @@ require (
 	github.com/hashicorp/go-multierror v1.1.1 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/josharian/intern v1.0.0 // indirect
-	github.com/klauspost/compress v1.18.5 // indirect
 	github.com/mailru/easyjson v0.7.7 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/mdlayher/socket v0.2.0 // indirect
 	github.com/mdlayher/vsock v1.1.1 // indirect
+	github.com/mitchellh/go-homedir v1.1.0 // indirect
 	github.com/mitchellh/mapstructure v1.4.3 // indirect
 	github.com/ncruces/go-strftime v0.1.9 // indirect
 	github.com/oklog/ulid v1.3.1 // indirect
+	github.com/opencontainers/go-digest v1.0.0 // indirect
+	github.com/opencontainers/image-spec v1.1.1 // indirect
 	github.com/opentracing/opentracing-go v1.2.0 // indirect
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
-	github.com/spf13/pflag v1.0.5 // indirect
+	github.com/spf13/pflag v1.0.10 // indirect
+	github.com/vbatts/tar-split v0.12.2 // indirect
 	github.com/vishvananda/netlink v1.1.1-0.20210330154013-f5de75959ad5 // indirect
 	github.com/vishvananda/netns v0.0.0-20210104183010-2eb08e3e575f // indirect
 	go.mongodb.org/mongo-driver v1.8.3 // indirect
 	golang.org/x/exp v0.0.0-20250620022241-b7579e27df2b // indirect
-	golang.org/x/mod v0.31.0 // indirect
-	golang.org/x/net v0.48.0 // indirect
-	golang.org/x/sync v0.19.0 // indirect
-	golang.org/x/text v0.32.0 // indirect
-	golang.org/x/tools v0.40.0 // indirect
+	golang.org/x/mod v0.35.0 // indirect
+	golang.org/x/net v0.53.0 // indirect
+	golang.org/x/sync v0.20.0 // indirect
+	golang.org/x/text v0.36.0 // indirect
+	golang.org/x/tools v0.44.0 // indirect
 	gopkg.in/yaml.v2 v2.4.0 // indirect
 	modernc.org/libc v1.66.3 // indirect
 	modernc.org/mathutil v1.7.1 // indirect