[Bug] Geo-replication V2 dedup watermark can be lost after target topic reload

### Problem

Persistent geo-replication V2 deduplication uses the source topic position as the target-side dedup watermark. For a replicated message, the source replicator adds `__MSG_PROP_REPL_SOURCE_POSITION=:`, and the target broker stores the latest replicated source position as:

- `_LID`
- `_EID`

This state is not normal producer sequence state. It is the target-side checkpoint used to identify whether a replayed source entry has already been persisted.

There are three related issues:

1. The geo V2 watermark is not recovered from replayed target entries.
2. The geo V2 watermark is stored as two separate snapshot keys, but `_LID` and `_EID` must be restored together.
3. The geo V2 watermark can be removed by normal producer inactivity cleanup, even though it is source-position state rather than producer lifecycle state.

### Failure window

1. Source replicator sends messages to the target.
2. Target persists the messages and updates the in-memory geo V2 dedup watermark.
3. The target topic is unloaded before the latest dedup snapshot includes the watermark.
4. The source replicator has not durably advanced its replication cursor yet.
5. Source reconnects and replays the same source entries.
6. Target reloads without the geo watermark and can write duplicates.

### Impact

This can produce duplicate messages on the target cluster during normal recovery/failover paths. It does not require client misuse or corrupted input. Source cursor replay is an expected recovery behavior, so target-side geo dedup state must survive topic reload and producer inactivity cleanup.

### Proposed fix

- Recover geo-replication V2 dedup watermarks while replaying the dedup cursor by reading `replicatedFrom` and `__MSG_PROP_REPL_SOURCE_POSITION` from persisted replicated messages.
- Store geo-replication watermark keys as complete `_LID/_EID` pairs in dedup snapshots.
- Keep geo-replication watermark state during producer inactivity cleanup.

A proposed fix is available in #25860.

Start by reading the proposed fix in #25860, then trace dedup cursor replay, persisted replicated messages, snapshot watermark keys, and producer inactivity cleanup. Done means replayed replicated entries recover the geo V2 watermark, _LID/_EID values are restored as pairs, and inactivity cleanup preserves that state across target topic reloads.

Written by the indexing model from the issue text.