Four resume-flow papercuts in `installer/install.sh` that hurt the
"interrupted install" path the most.
1. `--resume` with no state file is no longer silent.
The most common operator confusion: reboot the live ISO, forget
/tmp/ is tmpfs, re-run with --resume, watch the installer start
over from scratch without saying anything. Now: loud error, tmpfs
explanation, exit 1.
2. Validate the saved TARGET_DRIVE still exists on resume.
Live ISO USB sticks get unplugged between sessions, dev hosts
sometimes have non-deterministic /dev/sdX numbering. Without the
guard the install proceeds and fails with cryptic disko / mount
errors deep in execute_installation. Now we fail at load_state
with the actual reason and a clean recovery path.
3. Resume now shows what's being resumed.
`save_state` stamps an ISO-8601 timestamp; `load_state` prints
"Resumed from <path> (saved Xm ago)" plus a "Target: /dev/X → user
@ host" summary line. Lets the user Ctrl-C before any destructive
prompt fires if they're resuming onto the wrong machine.
4. `--help` documents the tmpfs limitation.
Saved state lives in /tmp/ which is tmpfs on the live ISO; --resume
only works within the same boot. The man-page now says so instead
of letting users discover it the hard way.
`format_age` is the one new helper — pretty-prints "Xs/Xm/Xh Ym/Xd"
relative to now, falls back to the raw timestamp if `date -d` can't
parse the input. shellcheck --severity=error passes.
Out of scope (potential future work):
- Persistent state across reboots (would need a writable USB / external
drive — chicken/egg with the installer setting up the only persistent
storage in the first place).
- `--show-state` flag to inspect a saved file without running.
- State-file schema versioning.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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