add broadcast exchange for cluster-wide settings push (#2)

[Esity]

Summary

• Add legion.cluster.control topic exchange for cluster-wide broadcast
• Per-node auto-delete queues bound on boot for settings push, feature flags, and killswitch
• New messages: ClusterSettings and ClusterKillswitch
• ClusterControl subscription actor routes by routing key to existing runners
• New runner methods: broadcast_settings and killswitch

Closes #2

Test Coverage

• Specs for exchange, queue, messages, actor, and runner methods
• bundle exec rspec — 162 examples, 0 failures
• bundle exec rubocop — zero offenses