docs: record Redis Cluster support as out of scope

.out-of-scope/redis-cluster.md

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+# Redis Cluster
+
+`TimersBroker` does not support Redis Cluster. Constructing it with a `RedisCluster` client
+raises `TypeError` with a message pointing at single-primary Redis, and Sentinel-managed
+primary/replica setups remain the supported high-availability option.
+
+## Why this is out of scope
+
+Every timer topic is stored as a pair of keys built by `TimerStore`:
+
+```python
+f"{timeline_key}:{full_topic}"  # sorted set: timer id -> activation time
+f"{payloads_key}:{full_topic}"  # hash: timer id -> envelope
+```
+
+Several operations touch both keys of a pair at once, and on Cluster that only works when both
+keys hash to the same slot:
+
+- the claim and commit Lua scripts, which take both keys as `KEYS[1]` and `KEYS[2]`
+- `schedule()`, which writes the sorted set and the hash in one `MULTI`
+- `cancel_all()`, which counts and unlinks both keys in one transaction
+
+With the current key names the two keys of a topic land in different slots, so publishing,
+consuming and cancelling would all fail with cross-slot errors. Fixing that means reshaping the
+key derivation with hash tags. That changes the storage layout for anyone with timers already in
+Redis, so it needs either a read-side migration path for existing keys or a hard break in a major
+version. That is a design project of its own, and nobody has asked for it: no user has reported
+running Cluster and being unable to adopt the package. Until then a clear error at construction
+is better than partial support.
+
+## If this is reopened
+
+- Users already choose `timeline_key` and `payloads_key`. If both prefixes share one hash tag
+  (for example `{timers}:tl` and `{timers}:pl`), every key hashes to the same slot, which might
+  make Cluster work with no storage change, at the cost of putting all timers on one shard.
+  Unverified: check that redis-py's `RedisCluster` routes the raw `EVALSHA` calls and
+  `transaction=True` pipelines correctly before relying on this.
+- Per-topic hash tags (`{full_topic}` inside both keys) would spread topics across shards but
+  change the key layout for existing deployments.
+- Revisit when a user reports running Redis Cluster and being blocked by this, or when
+  FastStream's `RedisClusterBroker` becomes the usual way its Redis integrations are deployed.
+
+## Prior requests
+
+- #66: "Decide whether to support Redis Cluster"