Fix LXST voice call interop with Python LXST/Columba

- Fix thread safety: defer Reticulum link callbacks (packet, link_closed)
  to call_update() which runs under LVGL lock, preventing crashes from
  concurrent LVGL access across cores
- Fix outgoing call signal handling: store link reference and re-register
  packet/link_closed callbacks in on_call_link_established so signals
  are actually received
- Fix call answer screen freeze: update UI before blocking audio init
  (I2S/ES7210/Codec2 setup) so screen renders immediately
- Fix audio direction: use startPlayback() (speaker RX) instead of
  startCapture() (mic TX) so received audio is actually heard
- Add msgpack wire format for LXST signalling and audio frames
- Add LXST IN destination for receiving calls + announce support
- Add incoming call UI (Answer/Reject buttons) on CallScreen
- Add path request before outgoing call link establishment
- Add LXST announce handler registration

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main.cpp

@@ -1130,6 +1130,11 @@ void setup() {
     setup_ui_manager();
     BOOT_PROFILE_END("ui_manager");
 
+    // Send initial LXST voice destination announce
+    if (ui_manager) {
+        ui_manager->announce_lxst();
+    }
+
     // Register delivered callback to update message status in storage and UI
     router->register_delivered_callback([](LXMF::LXMessage& msg) {
         INFO(">>> APP DELIVERED CALLBACK ENTRY");
@@ -1281,6 +1286,9 @@ void loop() {
                                         (ble_interface && ble_interface->online());
             if (router && has_online_interface) {
                 router->announce();
+                if (ui_manager) {
+                    ui_manager->announce_lxst();
+                }
                 last_announce = millis();
                 INFO("Periodic announce sent (interval: " + std::to_string(app_settings.announce_interval) + "s)");
             }