Remove Lima and socket_vmnet dependency from rootcell

[jimpudar]

Summary

• Remove the Lima rollback path and the local socket_vmnet package from the flake and runtime code
• Keep vfkit as the only supported VM backend and update the provider factory, instance state, and host-side helpers accordingly
• Clean up docs, tests, and comments so they only describe the vfkit flow

Testing

• bun run typecheck
• bun run lint
• bun test
• ./test

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