feat(valkey): Valkey Cluster support

[Arthurdw]

Summary

• Add `ValkeyStore::connect_cluster`; keys live in 256 hash-tag buckets so every script stays in one slot.
• The guard creates the session id first and mines the token into the same bucket for sharded stores; the store rejects misaligned writes.
• Subject-wide revoke is eventually consistent and crash-safe; a login racing a revoke is revoked or fails.

Notes

• Key layout changed, no migration: existing Valkey sessions are unreachable after upgrade (mass logout).
• Standalone purges now make 256 bucket calls.

Testing

• Conformance and multi-bucket tests against a cluster, plus e2e scenarios in cluster mode.
• CI runs a three-node cluster via `VALKEY_CLUSTER_URL`; local default is a single node (catches `CROSSSLOT`, not redirects).

[codecov]

Codecov Report

❌ Patch coverage is 95.42683% with 15 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/store/valkey.rs	93.56%	15 Missing ⚠️

📢 Thoughts on this report? Let us know!