baseline_command's Desktop checkpoint fallback list is hardcoded and will go stale

[rlnorthcutt]

Problem

`baseline_command` (lab.sh:392-414) resolves the recommended Desktop-suite
checkpoint by checking a hardcoded, literal list:

```bash
for candidate in desktop-e2e-v2 podman-ready clean; do
if "$ENGINE" snapshots "$vm" | grep -Fxq "$candidate"; then
printf '%s\n' "$candidate"
return
fi
done
```

Checkpoints are named with incrementing version suffixes (see
`docs/checkpoints.md`'s own example of cutting `desktop-e2e-v3`). Once a
newer checkpoint like `desktop-e2e-v3` exists, this code still resolves to
the older `desktop-e2e-v2` (or falls further back to `clean`) instead of
finding it — the wrong baseline gets silently selected, with no error to
signal the drift.

Suggested fix

Instead of an ordered literal list, fetch the snapshot list once and pick the
highest `desktop-e2e-` entry via version sort (`sort -V`), only falling
back to `podman-ready` then `clean` if no `desktop-e2e-` checkpoint
exists at all:

```bash
snapshots="$("$ENGINE" snapshots "$vm")"
newest="$(grep -E '^desktop-e2e-' <<<"$snapshots" | sort -V | tail -n1)"
```

Verified this resolves to `desktop-e2e-v2` against the existing
`tests/fake-engine.sh` fixture, so `tests/lab_test.sh` would keep passing
unmodified.

Files

• `lab.sh` (`baseline_command`, ~line 403)