Support multiple network interfaces per workload instance #442

Problem

Compute instances are limited to one network interface. This prevents network appliances and other multi-homed workloads from connecting to separate tenant networks—for example, a VyOS instance with management, WAN, and LAN interfaces.

Desired outcome

Customers can declare multiple named network interfaces on a workload. Each interface is attached to its selected network, receives independent addressing and status, and keeps a stable guest-visible identity across instance replacement.

One interface must be designated as primary for default routing and primary-IP presentation. Additional interfaces should be governed by project quota, while every instance continues to receive its first interface by default.

Acceptance criteria

• A workload can request more than one named network interface, subject to the selected runtime class's supported limit.
• Every declared interface is provisioned, attached, and reported independently.
• The primary interface and default-route behavior are deterministic.
• Adding, removing, or changing interfaces results in a safe instance rollout with no orphaned interface allocations.
• Quota can limit additional interfaces per project without hiding the API behind a feature flag.
• CLI output and documentation make all interfaces and their readiness visible.
• End-to-end coverage demonstrates a multi-homed VM or sandbox attached to at least three networks.

Dependencies

Requires corresponding multi-interface support in the cell networking path, including Multus injection, Galactic attachment lifecycle, and the active runtime provider.