v0.4.0

Completes the plugin connection lifecycle. v0.3.0 shipped onSubscribe but no onUnsubscribe, and PluginContext::connectionsOn() (read) with no write-side counterpart. A plugin could see a connection join a channel but not leave one short of a full disconnect, and could not evict a connection from a single channel without terminating the whole socket. Both gaps are now closed.

Added

• ConnectionLifecycle::onUnsubscribe(Connection, Channel) - fires when a connection leaves a channel via the explicit pusher:unsubscribe event.
• PluginManager::notifyUnsubscribe() - exception-isolated fan-out for the new hook.
• PluginContext::unsubscribe(Connection, string $channel) - removes a connection from one channel, leaving the socket and its other subscriptions intact. A direct action; it does not itself emit onUnsubscribe.

Changed

• ConnectionLifecycle gains a fourth method, onUnsubscribe. Breaking for existing implementers of the interface; the plugin system is one release old, so the contract is completed before it settles.
• EventHandler::unsubscribe() fires the onUnsubscribe lifecycle hook after removing the connection from the channel. A closing connection is still reported once through onClose, not as one unsubscribe per channel.

Suite grows to 317 tests.