feat(cli): ecrou provision bakes a command into a cache entry

prepare_provisioned and persist were sitting there with no consumer:
the machinery for "install once, every later boot carries it" that
one-shot machines cannot do on their own - packages, users, anything
short of a share - and no way to reach it.

ecrou provision IMAGE -- COMMAND is the missing surface. It boots a
machine over a freshly prepared rootfs, runs the command, and
persists the tree into the cache entry, marked provisioned; a marked
entry is extracted as-is on every later call, the command skipped.
A failing command leaves the entry unprovisioned for the next
attempt, and --no-cache re-downloads before re-provisioning.

The engine behind run splits out as vm::run_prepared: the boot half
of run, taking the rootfs the provisioner is about to persist. run
itself is prepare plus that.

One cache fix the provision flow surfaced: a digest refresh replaced
the tarball but left the old .provisioned marker, so a fresh pristine
image would present as provisioned and skip provisioning for good.
The marker now dies with the entry it belonged to.

SPEC.md

@@ -269,6 +269,7 @@ composable. A boot failure leaves the files absent.
 
 ```
 ecrou run [OPTIONS] <IMAGE> [-- COMMAND...]
+ecrou provision [OPTIONS] <IMAGE> [-- COMMAND...]
 ecrou probe
 ```
 
@@ -283,6 +284,17 @@ ecrou probe
 | `--no-cache` | false | Fresh download, ignore the cache |
 | `-v, --verbose` | false | Diagnostics; raises the libkrun log level |
 
+`ecrou provision IMAGE -- COMMAND` bakes a command into the image's
+cache entry: the command runs in a machine over a freshly prepared
+rootfs and the result is persisted, so every later boot of that image
+carries what the command installed — packages, users, whatever a
+one-shot boot cannot keep. A marked entry is reused as-is (the
+command is skipped), `--no-cache` re-downloads and re-provisions, and
+a failing command leaves the entry unprovisioned for the next attempt.
+The marker dies with the entry: a digest refresh drops it, so a moved
+upstream image is provisioned again, never silently served pristine
+under an old marker.
+
 `ecrou probe` reports what a run would find: KVM working (open, ioctl
 version, nested-virt state), `libkrun.so.1` loadable, the cache
 directory writable. It is the confinement diagnostic and the support