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/runner.rs

@@ -56,15 +56,27 @@ pub fn run(socket_name: &str, cdylib_path: &Path) -> Result<(), Box<dyn std::err
 
     if version >= 4 {
         let client = V4Client::connect_handshake(socket_name, &token)?;
+        // Give the plugin a chance to set up worker threads / cache the request
+        // sender before the host starts dispatching user commands.
+        invoke_on_load(&mut *plugin, &client, &interactive);
         run_v4(&mut *plugin, &interactive, client)
     } else {
         let client = IpcClient::connect(socket_name)?;
         eprintln!("[runner] connected to host");
         client.send_handshake(&token)?;
+        // V2/V3 plugins receive HostApi only during dispatch, so on_load is not
+        // meaningful there; skip it to keep the old protocol unchanged.
         run_v3(&*plugin, &interactive, client)
     }
 }
 
+fn invoke_on_load(plugin: &mut dyn BuiltinPlugin, client: &V4Client, interactive: &InteractiveRegistry) {
+    let mut proxy = client.plugin_host_api(0, interactive.clone());
+    let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+        plugin.on_load(&mut proxy)
+    }));
+}
+
 fn run_v3(
     plugin: &dyn BuiltinPlugin,
     interactive: &InteractiveRegistry,