v0.3.6

Added

• legion.cluster.control topic exchange for cluster-wide broadcast (durable, topic type)
• Per-node auto-delete queue legion.cluster.control.<name> bound to the exchange on boot
• ClusterSettings message: publishes settings changes with configurable routing key to the cluster control exchange
• ClusterKillswitch message: publishes emergency extension block via routing key settings.extensions.blocked
• ClusterControl subscription actor: subscribes to the per-node cluster control queue, routes to existing runners by routing key
• Runners::Node#broadcast_settings(settings:, routing_key: 'settings', restart: false): sends settings to all nodes via the cluster control exchange
• Runners::Node#killswitch(extension:): blocks an extension cluster-wide with immediate reload
• 51 new specs covering all new components (162 total, 0 failures)