fix(runtime): make backups restore-safe

[MarcStdt]

Implements HIG-71's runtime-safe backup and restore foundation.

• carries the installed release descriptor through automated backup and restore workers
• stops and keeps runtimes quiescent throughout backup/restore; serializes Start, Ensure, Update, Stop, and Delete against the operation
• resumes a maintenance-stopped queue before the post-backup/restore start, so restart work actually runs
• stages restore content on the runtime volume and transactionally rolls back post-mutation failures
• leaves a runtime stopped with a clear error when rollback itself fails; otherwise restarts the prior runtime after a recoverable failure
• maps host.docker.internal to Docker's host gateway for worker containers on all supported host OSes

Depends on #122 for the shared OCI transfer option.

Validation:

• make test-integration-docker — PASS (3/3, 50.517s): CLI lifecycle, volume-worker backup/restore, and coldstarter
• focused runtime/core, Docker backend, Kubernetes backend, and mock checks passed

The Docker suite validates the runtime layer only; full platform lifecycle verification remains tracked separately.

[druid-infra]

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