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Index Lifecycle: Zero-Downtime Reindex with Aliases, Index Templates, Shard Sizing and ILM

If the application talks to an alias instead of a real index name, a mapping change is not an outage but a new index from a template, a _reindex and one atomic _aliases request. With real output: remove_index on the first switch, a half-applied alias request and must_exist, version_type external, replaying changes that arrive during the reindex from a checkpoint, require_alias, choosing the shard count, ILM with a data stream, and snapshot/restore.