design: propose scalable Gantry peer discovery

[Vaibhav Patel (vpatelsj)]

Summary

• explain why complete Pod and Node membership does not scale to 100,000 Gantry agents
• propose fixed Kubernetes Lease slots for first contact and DHT routing after bootstrap
• retain provider lookup for warm content and replace HRW with closest-peer selection for cold pulls
• document failure behavior, scale properties, rollout, and validation requirements

Testing

• documentation only