registry: add backend/frontend registries and plugin loader

examples/trx-plugin-example/Cargo.toml

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+# SPDX-FileCopyrightText: 2025 Stanislaw Grams <stanislawgrams@gmail.com>
+#
+# SPDX-License-Identifier: BSD-2-Clause
+
+[package]
+name = "trx-plugin-example"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+trx-backend = { path = "../../src/trx-backend" }
+trx-core = { path = "../../src/trx-core" }
+trx-frontend = { path = "../../src/trx-frontend" }
+tokio = { workspace = true, features = ["full"] }
+tracing = { workspace = true }

examples/trx-plugin-example/README.md

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+# 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.

examples/trx-plugin-example/src/lib.rs

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+// SPDX-FileCopyrightText: 2025 Stanislaw Grams <stanislawgrams@gmail.com>
+//
+// SPDX-License-Identifier: BSD-2-Clause
+
+use std::net::SocketAddr;
+
+use tokio::sync::{mpsc, watch};
+use tokio::task::JoinHandle;
+use tracing::info;
+
+use trx_backend::{register_backend, RigAccess};
+use trx_core::{DynResult, RigRequest, RigState};
+use trx_frontend::{register_frontend, FrontendSpawner};
+
+const BACKEND_NAME: &str = "example";
+const FRONTEND_NAME: &str = "example-frontend";
+
+/// Entry point called by trx-bin when the plugin is loaded.
+#[no_mangle]
+pub extern "C" fn trx_register() {
+    register_backend(BACKEND_NAME, example_backend_factory);
+    register_frontend(FRONTEND_NAME, ExampleFrontend::spawn_frontend);
+}
+
+fn example_backend_factory(_access: RigAccess) -> DynResult<Box<dyn trx_core::rig::RigCat>> {
+    Err("example plugin backend not implemented".into())
+}
+
+struct ExampleFrontend;
+
+impl FrontendSpawner for ExampleFrontend {
+    fn spawn_frontend(
+        _state_rx: watch::Receiver<RigState>,
+        _rig_tx: mpsc::Sender<RigRequest>,
+        _callsign: Option<String>,
+        listen_addr: SocketAddr,
+    ) -> JoinHandle<()> {
+        tokio::spawn(async move {
+            info!("example frontend loaded at {} (no-op)", listen_addr);
+        })
+    }
+}