registry: add backend/frontend registries and plugin loader

examples/trx-plugin-example/README.md

@@ -0,0 +1,19 @@
+# trx-plugin-example
+
+This is a minimal shared-library plugin that registers a backend and frontend.
+The backend is a stub that returns an error; the frontend is a no-op spawner.
+
+Build:
+
+```bash
+cargo build -p trx-plugin-example --release
+```
+
+Install (example):
+
+```bash
+mkdir -p plugins
+cp target/release/libtrx_plugin_example.* plugins/
+```
+
+Run `trx-bin` with `TRX_PLUGIN_DIRS=./plugins` to discover the plugin.