feat(plugins): add BuiltinPlugin::on_load lifecycle callback

OpenCADStudio/plugins#901 (comment)

- Add BuiltinPlugin::on_load(&mut self, host: &mut dyn HostApi) with a default
  no-op implementation so existing plugins remain source-compatible.
- Invoke on_load once for V4 plugins immediately after the runner connects
  and before entering the request loop, giving plugins a HostApi surface to
  cache plugin_request_sender() or set up worker threads before any dispatch.
- V2/V3 plugins are unchanged: HostApi is only available during dispatch.

crates/ocs_plugin_api/src/host.rs

@@ -298,6 +298,16 @@ pub trait BuiltinPlugin: Send + Sync {
     /// the V4 runner guarantees it is only called for plugins that report
     /// 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.
+    ///
+    /// 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.
+    fn on_load(&mut self, _host: &mut dyn HostApi) {}
 }
 
 /// A point-driven interactive command a plugin starts via