Add ecr::rootfs with the full rootfs lifecycle behind the library:
- RootfsCache::prepare resolves an image reference, downloads through
the tarball cache (with the OCI :latest digest freshness check) and
extracts into a scratch directory tracked by PreparedRootfs.
- PreparedRootfs::persist packs the current rootfs back into its cache
entry (compressed to match the entry's extension, symlinks and
permissions preserved) and marks it with a .provisioned sidecar.
- RootfsCache::prepare_provisioned composes both into the hot-cell
flow: provision once, and every later call sharing the cache skips
the download and the provisioning step.
extract: an "oci-" cache entry without a layers.manifest is a
persisted provisioned rootfs; extract it as a plain archive.
The CLI now drives prepare and drops its inline cache/orchestration
code and the dirs/tempfile dependencies. The binfmt check moves ahead
of the download so foreign-arch runs fail before pulling an image.
Add ecr::exec: run a command inside a prepared rootfs in fresh
user/PID/mount/UTS namespaces, with the caller composing the full
environment (ExecOptions::env), the bind targets (BindTarget, with
explicit absolute mount points inside the rootfs) and the target
architecture (binfmt_misc is verified for foreign arches).
mount::setup_mounts now takes &[BindTarget] instead of parallel
read-only/read-write path lists; chroot::run_chroot takes the envp and
a resolved working directory, and chroot::default_env composes the
previous hardcoded environment as a starting point for callers.
The CLI maps its flags onto the new API; behavior is unchanged
(overlay at /root/<basename>, rw bind at /mnt/<basename>, cwd default).
The root package becomes a virtual workspace: `crates/ecr` holds the
library (package name `ecr`) and `crates/ecr-cli` the command line
front-end, which keeps installing the `ecr` binary. No behavior change.
The library must not depend on CLI types, so mount::setup_mounts now
takes the `no_bind` flag instead of a `&Args`.
vhaudiquet