Redis cluster support Phase 6: cluster e2e CI job + static cross-entity-pipeline lint rule + rollout

[zhchxiao123]

What to build

Add a local Cluster integration environment (Maven test profile or docker-compose bringing up a minimal 3-master/3-replica Redis Cluster) wired into CI as an optional/slow job.

Add a static check (Checkstyle custom rule, or a simple build-time scan) that flags any new code sequence mixing two different Constants.java key-factory-method outputs with different parameters into the same multi-key call, unless that combination is already registered as a same-entity group sharing a hash tag — this guards against reintroducing the cross-entity pipeline bug class fixed in Phase 3.

Update this repo's docs on cluster mode support.

Coordinate gradual/staged production rollout of cluster mode for any deployments that want it — distinct from the key-schema migration in Phase 2b, which can happen while still running standalone.

Acceptance criteria

• Local Cluster environment added, wired as an optional/slow CI job
• Static check added and passing on current codebase, catching newly introduced cross-entity multi-key operations in CI
• Repo docs updated to describe REDIS_MODE=cluster usage and prerequisites
• Gradual rollout plan documented for any deployment that wants cluster mode enabled, with a rollback path

Blocked by

#13, #14