feat(cluster): route sharded Pub/Sub by slot

[mcollina]

Summary

• route SSUBSCRIBE and SUNSUBSCRIBE to the primary that owns the channel slot
• keep independent lazy sharded subscriber connections per active primary
• preserve subscriptions across topology refreshes, MOVED responses, reconnects, and slot migration
• support client-wide subscription counts and zero-argument SUNSUBSCRIBE
• clean up stale subscriptions, connections, listeners, retries, and timers
• keep classic Pub/Sub on the existing cluster subscriber

Closes #13.

Integration coverage

Adds a dedicated test-valkey-cluster GitHub Actions job. It starts three Valkey primaries and three replicas from the pinned valkey/valkey:8.0.9 image, then verifies real SSUBSCRIBE/SPUBLISH delivery for channels owned by different primaries.

The same test runs locally with:

npm run test:integration:valkey-cluster

Validation

• npm run build
• npm run test:tsd
• targeted sharded Pub/Sub tests: 10 passing
• Docker Valkey Cluster integration: passing
• npm run lint: no errors; 12 pre-existing warnings
• full JS suite: 425 passing; the existing localhost IPv6 assertions fail on this IPv4-resolving host and cause two cascading test failures

Upstream attribution

ClusterSubscriberGroup and ShardedSubscriber adapt work from redis/ioredis PRs #1956, #2013, #2043, #2060, and #2090.

The implementation commit includes Co-authored-by attribution for David Maier, Tihomir Krasimirov Mateev, Pavel Pashov, and Hristo Temelski.