[refactor](trx-rs): inject runtime contexts for io paths

Phase 3: replace frontend/backend hot-path globals with explicit runtime/registration context wiring while keeping plugin compatibility adapters.

Co-authored-by: Codex <codex@openai.com>,
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-client/trx-frontend/trx-frontend-rigctl/src/lib.rs

@@ -4,6 +4,11 @@
 
 pub mod server;
 
+pub fn register_frontend_on(context: &mut trx_frontend::FrontendRegistrationContext) {
+    use trx_frontend::FrontendSpawner;
+    context.register_frontend("rigctl", server::RigctlFrontend::spawn_frontend);
+}
+
 pub fn register_frontend() {
     use trx_frontend::FrontendSpawner;
     trx_frontend::register_frontend("rigctl", server::RigctlFrontend::spawn_frontend);