Add CUDN Localnet fabric integration guide

[danmanor]

Summary

• Guide for connecting KubeVirt VMs to a physical fabric using CUDN Localnet topology with VLAN tagging on br-ex
• Covers single-NIC trunk mode: node management (untagged) + VM tenant traffic (tagged VLANs) on the same switch port
• Step-by-step setup: NMState, NNCP bridge-mappings, CUDN with Localnet topology, VM creation, jump pod SSH access
• Multi-cluster IP partitioning via excludeSubnets
• Verified test results across all connectivity combinations:
  • Same virtual network, same cluster (L2, TTL=64)
  • Same virtual network, cross-cluster (L2, TTL=64)
  • Different virtual networks, same VPC/VRF (L3, TTL=63, requires manual routes)
  • Different virtual networks, different VPCs/VRFs (isolated)
  • SNAT via fabric vs pod network (different public IPs)
• Mermaid diagrams for all networking flows and OVN internal architecture
• Troubleshooting section including masquerade subnet collision (10.0.2.0/24) and virtctl ssh breakage

Test plan

All tests were run on 2 SNO clusters (OCP 4.21 and 4.22) on a managed fabric with 2 virtual networks (VLAN 3 and VLAN 5). Results documented in the guide.

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📥 Commits

Reviewing files that changed from the base of the PR and between 010cba4 and 8c427bc.

📒 Files selected for processing (2)

• networking/cudn-localnet-fabric-integration/README.md
• networking/cudn-localnet-fabric-integration/diagrams.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch cudn-localnet-fabric-integration

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