Multi-box scaling and warm-handoff moves for bare-metal regions

Bare-metal Kura regions can now span multiple boxes per region. Each account’s traffic is routed via a per-account DNS record to the specific box hosting its pods, instead of round-robining across every box in the region. When an account needs to be moved off a hot box, a second instance is provisioned on the destination box, warmed up over the account peer plane while the source keeps serving, and then promotion swaps host ownership atomically. The cutover drains existing connections and fails open to the origin, so accounts can be rebalanced or scaled across boxes without a cold-cache dip.