docs: describe the image-backed rootfs

The design paragraphs follow the code: the rootfs crosses as a raw
ext4 image built from the tarball, ownership is real inside the
machine, provisioning rides debugfs, and the control file mints the
exit code because the engine's own channel is virtiofs-root only.
The confinement budget admits the e2fsprogs children and cp, the
snap section stages e2fsprogs and names the blk feature, and the
boot-economics row counts an image copy instead of an extraction.

SPEC.md

@@ -66,8 +66,10 @@ Everything the host process may do, and nothing else:
 
 - fs: read, write, stat, rename, unlink, mkdir, fallocate under the
   cache directory and the share roots it was given, plus the scratch
-  rootfs TempDir
-- exec: itself; libraries via `dlopen` (`libkrun.so.1`, libkrunfw)
+  directory holding the run's image copy
+- exec: itself; libraries via `dlopen` (`libkrun.so.1`, libkrunfw);
+  e2fsprogs helpers (`mkfs.ext4`, `debugfs`) as child processes, and
+  `cp` for the per-run image copy
 - `/dev/kvm`: open, ioctl, mmap
 - net: TCP and UDP client sockets; with TSI these carry the guest's
   connections too, proxied host-side
@@ -93,9 +95,13 @@ are the shape. A guest agent and long-lived machines are a Later row.
 
 Ephemeral rootfs, durable workspace. The base system is a scratch copy
 of the image; it exists only for the run and dies with it. Persistence
-crosses a share or it does not happen. No initramfs and no cpio
-builder: the rootfs crosses as a virtiofs share, never as a packed
-archive.
+crosses a share or it does not happen. The rootfs crosses as a raw
+ext4 image: mke2fs builds a cache image straight from the image
+tarball, the run copies it into a scratch directory, and the engine
+attaches it as a virtio-blk disk the init blob mounts at `/`. No
+initramfs and no cpio builder, and no extraction to a host tree —
+an unprivileged extract cannot preserve a tar's owners, and the
+tarball-to-image pipeline never needs one.
 
 **Workspace shares.** `-v HOST:GUEST[:ro]`, one virtiofs share per
 entry. The host side is plain fs; the guest init mounts each share at
@@ -103,13 +109,18 @@ its guest path. There is no implicit share: an empty machine is the
 default, and the consumer chooses what crosses — a sandbox runtime
 does not silently attach the caller's cwd.
 
-Ownership needs no mapping layer. The guest runs as root inside its
-own kernel, and libkrun's simplified virtiofs semantics report the
-requesting guest uid for ownership while storing permission bits
-host-side; writes through a share land owned by the host user.
+Ownership is real inside the machine. The image bakes the ownership
+the archive records — mke2fs reads uid and gid from the tarball — and
+the guest runs as root on a real filesystem, so `chown` inside the
+machine works, including recursive ones that a virtiofs share owned by
+the host user refuses with EPERM. Shares remain the honest exception:
+the host side belongs to the host user, so writes through a share land
+owned by the host user and a guest chown there fails exactly as it
+would on the host.
 
-**The guest init.** A small script provisioned into the scratch rootfs
-for every run: it mounts each share, cds to the workdir, runs the
+**The guest init.** A small script written into the image copy
+through debugfs for every run: it mounts each share, cds to the
+workdir, runs the
 payload, and writes the exit code to the control file. It never execs
 — the init must outlive the payload to report it — and has no
 share-less special case: the same contract covers a bare shell run.
@@ -119,18 +130,20 @@ runs through the image's shell as `sh -c`. Ending the machine is not
 the script's job: when it exits, the init blob reports the code and
 the VMM powers the machine off.
 
-**Exit codes.** Only the guest init can mint a guest exit code: it
-reports the payload's code through the control file and the VMM exits
-the machine child with it. A signal on the machine child means the VMM
-or the engine died — the payload never reported — and `run` fails with
+**Exit codes.** Only the guest init can mint a guest exit code, and
+the control file is how: the init writes the payload's code before it
+exits, and the run reads it back out of the image after the machine
+dies. The file outranks the machine child's own exit code — with a
+block root the engine's exit-code channel (an ioctl through the
+virtiofs root device) never fires, so the child's code is machinery
+talk, not a report. A signal on the machine child means the VMM or the
+engine died — the payload never reported — and `run` fails with
 an engine-failure error, never a 128+signal guess at a guest code.
 125 and 126 are reserved by the machinery itself: the child bails
 with 125 on a boot failure and the init refuses with 126 when a share
 would not mount or the workdir is missing. Ending on either without a
-control-file code is an error naming the cause; a payload that
-genuinely exits either code reported through the file first, and the
-file is the arbiter. The control file remains the boot diagnostic in
-every other death.
+control-file code is an error naming the cause. The control file
+remains the boot diagnostic in every other death.
 
 **Cancellation is a kill.** The VMM lives in a child of the ecrou
 process carrying PR_SET_PDEATHSIG, so a killed consumer kills the
@@ -241,9 +254,10 @@ stop:
 - **T4, kill chain.** SIGKILL ecrou-cli mid-run; the VM is gone with
   the process, no stray threads or KVM VMs, verified by scanning the
   process table and `/dev/kvm` users.
-- **T5, boot economics.** Alpine, cache-warm: extract plus boot lands
-  under 1.5 s to first guest output. Larger images report their numbers
-  unblocked; the shared-ro-rootfs row in Later is the fix if they hurt.
+- **T5, boot economics.** Alpine, cache-warm: image copy plus boot
+  lands under 1.5 s to first guest output. Larger images report their
+  numbers unblocked; the shared-ro-rootfs row in Later is the fix if
+  they hurt.
 - **T6, no-mount audit.** `strace -f -e trace=mount,umount2,unshare,
   setns,pivot_root,open_tree,move_mount,fsopen,fsmount` over a full run
   with shares: zero hits. This test is the founding rule.
@@ -256,7 +270,9 @@ Confinement: strict. Plugs: `network`, `home`, `kvm`. The manual
 `snap connect` for kvm is accepted until auto-connect is a store
 request and a later product decision.
 
-The snap stages `libkrun.so.1` and libkrunfw next to the binary; the
+The snap stages `libkrun.so.1` (built with the `blk` feature: the
+virtio-blk device is what carries the rootfs) and libkrunfw next to
+the binary, plus e2fsprogs for `mkfs.ext4` and `debugfs`; the
 kernel ships inside libkrunfw, so there is nothing else to fetch or
 stage. No Ubuntu release publishes a `libkrun.so.1` package today, and
 libkrunfw5 exists only in the current development series' universe,
@@ -274,9 +290,9 @@ build is deleted.
 
 - **QEMU engine.** The TCG fallback for machines without KVM. The
   `RunOptions` API does not change; the engine choice is the probe's.
-- **Shared read-only rootfs.** Serve the cache rootfs directly, ro,
-  with a guest-side overlay for writes: no extraction per run. The
-  correct-and-slow v1 posture graduates into this.
+- **Shared read-only rootfs.** Serve the cache image directly, ro,
+  with a guest-side overlay for writes: no copy per run. The
+  copy-and-throw-away v1 posture graduates into this.
 - **Guest agent, long-lived machines.** Exec without a boot; the shape
   to grow into when per-command VMs stop being right.
 - **A build story for hosts that cannot build.** Provision was cut