feat: initial lora-dongle firmware scaffold

Transparent LoRa-over-USB dongle firmware supporting Heltec V3 (ESP32-S3)
and RAK 4631 (nRF52840) boards. Actor-model architecture with typed
channels between USB, radio, and display tasks.

- Protocol: Command/Response enums with COBS+postcard framing design
- Board abstraction: auto-discovered via build.rs Jinja2 codegen
- Radio task: SX1262 state machine (Idle → Receiving ↔ Transmitting)
- Display task: optional SSD1306 dashboard via StatusWatch
- RAK 4631: fully wired SPI/USB/I2C peripherals, clippy-clean
- Heltec V3: structure in place, awaits espup Xtensa toolchain

src/main.rs

@@ -0,0 +1,39 @@
+#![no_std]
+#![no_main]
+
+mod board;
+mod channel;
+mod display;
+mod error;
+mod protocol;
+mod radio;
+mod usb;
+
+use embassy_executor::Spawner;
+
+use crate::channel::{CommandChannel, ResponseChannel, StatusWatch};
+
+cfg_if::cfg_if! {
+    if #[cfg(feature = "rak_4631")] {
+        use panic_probe as _;
+    } else if #[cfg(feature = "heltec_v3")] {
+        use esp_backtrace as _;
+    }
+}
+
+static COMMANDS: CommandChannel = CommandChannel::new();
+static RESPONSES: ResponseChannel = ResponseChannel::new();
+static STATUS: StatusWatch = StatusWatch::new();
+
+#[embassy_executor::main]
+async fn main(spawner: Spawner) {
+    let board = board::Board::init();
+    let (radio, usb, display) = board.into_parts();
+
+    spawner.spawn(radio::radio_task(radio, &COMMANDS, &RESPONSES, &STATUS)).unwrap();
+    spawner.spawn(usb::usb_task(usb, &COMMANDS, &RESPONSES)).unwrap();
+
+    if let Some(dp) = display {
+        spawner.spawn(display::display_task(dp, &STATUS)).unwrap();
+    }
+}