feat(rootfs): cross the rootfs as an ext4 image on a block device

The rootfs was a virtiofs share over an extracted tree, and that
showed: the tree's owners were the host user's, because an
unprivileged extract cannot chown to a tar's uid, and every chown
inside the machine failed with EPERM against the host side.

The rootfs crosses as a raw ext4 image now. mke2fs builds the cache
image straight from the entry tarball (-d takes gz, xz and zst, and
bakes each entry's recorded uid, gid and mode), the run copies it
into a scratch directory with cp --reflink=auto --sparse=auto, and
the engine attaches it with krun_add_disk +
krun_set_root_disk_remount; the init blob mounts /dev/vda at / and
chroots. The OCI layer bundle merges into one plain tar first, with
whiteouts applied across layers and headers passed verbatim, so
registry images keep their ownership too.

Per-run provisioning rides debugfs: the generated init lands at
/ecrou-init, the host resolver overwrites /etc/resolv.conf (unlink
first - resolver-bearing images ship it as a symlink), a stale
control file goes. The exit-code control file now outranks the
machine child's exit code: with a block root the engine's exit-code
channel (an ioctl through the virtiofs root device) never fires, so
decode_status reads the file first and treats the child's code as
machinery talk. Shell detection and the rare control-file read are
debugfs stats, no extracted tree to walk.

extract.rs keeps what the image build consumes (the tar decoder and
the OCI merge) and drops the extraction-to-disk path it once carried;
the module is private now. The engine needs the blk feature from here
on: the snap and the CI engine build follow in their own commits.

AGENTS.md

@@ -38,10 +38,12 @@ Rules:
     (`crates/ecrou/src/rootfs.rs`)
   - `cache` - the rootfs entry store: naming, locking, freshness,
     layer pointers (`crates/ecrou/src/cache.rs`)
-  - `pack` - rootfs tarball packing (`crates/ecrou/src/pack.rs`)
+  - `image` - ext4 image building and inspection
+    (`crates/ecrou/src/image.rs`)
   - `distro` - distro definitions and mirrors (`crates/ecrou/src/distro.rs`)
   - `download` - image download (`crates/ecrou/src/download.rs`)
-  - `extract` - rootfs extraction (`crates/ecrou/src/extract.rs`)
+  - `extract` - tar decoding and the OCI layer merge feeding the image
+    build (`crates/ecrou/src/extract.rs`)
   - `registry` - OCI registry client (`crates/ecrou/src/registry.rs`)
   - `utils`, `error`, `progress` - shared helpers
   - `deps` - dependency additions/bumps (manifests, lockfile)