Multi-box support for bare-metal Kura regions with per-account routing and warm-handoff rebalancing

Bare-metal Kura regions can now scale to multiple boxes per region. Each account’s traffic is routed to the specific box hosting its pods via a per-account DNS record, so accounts no longer round-robin across boxes that do not host their data. Operators can also rebalance accounts between boxes with a warm-handoff: the destination box warms its cache from the source over the peer plane, then traffic is atomically swapped once it has caught up, avoiding the cold-cache dip you would see with a cold cutover.