plugin: export Qt plugin entry symbols from cdylib via --export-dynamic

plugin/build.rs

@@ -1,6 +1,13 @@
 use cxx_qt_build::{CxxQtBuilder, PluginType, QmlModule};
 
 fn main() {
+    // Rust cdylib uses a version script that hides everything not declared
+    // `#[no_mangle] pub extern "C"`. cxx-qt's QQmlEngineExtensionPlugin entry
+    // points (qt_plugin_instance, qt_plugin_query_metadata_v2) come from
+    // compiled C++ and would otherwise be stripped to local visibility, so Qt's
+    // plugin loader finds nothing exported. --export-dynamic re-exposes them.
+    println!("cargo:rustc-link-arg=-Wl,--export-dynamic");
+
     CxxQtBuilder::new_qml_module(QmlModule::new("NovaStats").plugin_type(PluginType::Dynamic))
         .files(["src/system_stats.rs", "src/cpu_service.rs"])
         .build();