Gapless Kura cache deploys with no write stall

Cache regions now run a warm standby replica on the same box with automatic failover. During a rolling deploy the controller fails the cache Service over to a Ready standby that has already replicated the latest state via the async outbox, so writes and serving no longer stall while a primary rolls. Only a brief window of misses on the very newest artefacts is possible, and those regenerate naturally on the next build.