Engine snapshots: hawser snapshot save/restore/clone the engine state #122

Save, branch, and roll back the whole engine state — images and volumes — as named checkpoints. Docker Desktop has no engine-state snapshotting at all.

What it does

• hawser snapshot save <name> — checkpoint the engine's VHDX (all images + volumes) to a named, checksummed archive
• hawser snapshot restore <name> — roll the engine back to it
• hawser snapshot list / delete
• refuses while containers are running (reuse the idle busy-probe), stops the engine cleanly first (never wsl --shutdown), operates only on Hawser's own distro

Why it's useful

• "Set up my dev environment, snapshot it, trash it during a risky test, restore in seconds."
• Reproducible starting points for demos and CI, complementing the reproducible install (hawser.lock, air-gap bundle).

Risk / notes

Touches VHDX-level operations (export/import or copy of the disk). Non-destructive by contract (snapshots are additive; restore is explicit and confirmed), but needs careful testing — flag for review before merge. Overlaps mechanically with #64's data-dir relocation; may share code.