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• UNKNOWN: The partition has no cached state (broker just became leader, state not loaded yet). Not an explicit API-driven state, just the absence of state.
• PREPARING: The coordinator for this partition detects via onMetadataUpdate that it leads a mirror partition. It fetches last mirrored offsets from the source cluster and truncates logs to align the local log with the source.
  Valid from: null, UNKNOWN, STOPPED, FAILED.
• MIRRORING: All ISR members have completed truncation. A MirrorFetcherThread is started to continuously replicate records from the source cluster. Valid from: PREPARING, PAUSED.
• PAUSING: Triggered by PauseMirrorTopics API (appends .paused suffix to mirror.name config). The system removes fetchers for the affected partitions. Valid from: MIRRORING only.
• PAUSED: Fetchers have been removed. The partition stays read-only with no active fetchers and no metadata sync (configs, consumer groups, ACLs). On resume, transitions directly to MIRRORING (fetchers resume from local LEO, no truncation needed). Valid from: PAUSING only.
• STOPPING: Triggered by RemoveTopicsFromMirror API (user wants to fail over) or topic deletion on the source. The system records the last mirrored offset to the internal topic. Valid from: PREPARING, MIRRORING, PAUSING (race guard, TODO: remove once shared queue serializes transitions), PAUSED.
• STOPPED: Last mirrored offsets have been persisted. The topic becomes writable on the destination cluster (the mirror fetcher is removed and the read-only flag is cleared). Valid from: STOPPING only.
• FAILED: An error occurred. Can transition back to PREPARING to retry. Valid from: any state.

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