fix(plugins): gate on_load behind API v5

BREAKING CHANGE: plugins using on_load must target API v5 and declare api_version = 5.

Refs #901

crates/ocs_plugin_api/src/host.rs

@@ -299,14 +299,12 @@ pub trait BuiltinPlugin: Send + Sync {
     /// API major 4 or newer.
     fn on_notification(&mut self, _command_id: Option<u64>, _notification: HostNotification) {}
 
-    /// Lifecycle callback invoked once after the plugin has been loaded and its
-    /// manifest/ribbon have been accepted by the host. The plugin receives the
-    /// active document tab's `HostApi` so it can set up worker threads or cache
-    /// `plugin_request_sender()` before any user command runs.
+    /// Lifecycle callback invoked once after the runner connects. The plugin
+    /// receives the active document tab's `HostApi` so it can set up worker
+    /// threads or cache `plugin_request_sender()` before any user command runs.
     ///
-    /// Added after API v4. The default implementation does nothing, so existing
-    /// plugins remain source-compatible. The runner only calls this for plugins
-    /// that report API major 4 or newer.
+    /// Added in API v5. The runner only calls this for plugins that report API
+    /// major 5 or newer.
     fn on_load(&mut self, _host: &mut dyn HostApi) {}
 }