Adds a Freezer ABC (backend/freeze.py) that encapsulates the
stop-commit-mark-preserved flow for all backends, following the same
pattern as BottleBackend. Each backend gets its own Freezer subclass:
DockerFreezer — docker commit
MacosContainerFreezer — container export + image rebuild; prompts
to stop if the container is running
SmolmachinesFreezer — smolvm pack create --from-vm
The base class owns write_committed_image, mark_preserved, and the
resume hint. Subclasses implement _freeze() and optionally override
_export_hint() for migration instructions.
Freezer.commit(agent, bottle) is the primary entry point for use
within a live launch context. Freezer.commit_slug(slug) is a
convenience wrapper for cmd_commit, which no longer branches on
backend names itself.
get_freezer(backend_name) is the factory, analogous to
get_bottle_backend(). CommitCancelled is raised by MacosContainerFreezer
when the user declines the stop prompt; cmd_commit catches it and
returns 0.
