feat(vz-runner): bridge guest shims to the host broker

[nappa85]

Why

An isolated VZ guest cannot connect directly to the host Unix broker socket. The runner needs a lifecycle-owned VSOCK bridge that preserves custody and fails before boot when the upstream broker is unavailable.

What Changed

• Add a VSOCK-to-Unix broker bridge plan and Virtualization.framework socket-device installation.
• Preflight the host broker connection before VM startup.
• Forward bytes bidirectionally without interpreting the broker protocol.
• Reap completed connections during normal operation and shut down active workers with the VM.
• Add forwarding, preflight, reaping, and teardown coverage.

Risks / Notes

• Descriptor ownership and worker shutdown are security-sensitive lifecycle boundaries.
• Virtualization.framework behavior still requires native macOS CI and runtime validation.

AI Assistance

Prepared with openai/gpt-5.6-sol.