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/usb/task.rs

@@ -0,0 +1,26 @@
+use embassy_executor::task;
+
+use crate::board::UsbParts;
+use crate::channel::{CommandChannel, ResponseChannel};
+
+#[task]
+pub async fn usb_task(
+    parts: UsbParts,
+    commands: &'static CommandChannel,
+    responses: &'static ResponseChannel,
+) {
+    let _driver = parts.driver;
+    let _ = commands;
+    let _ = responses;
+
+    // TODO: build embassy_usb CDC-ACM class from driver
+    // TODO: COBS-framed postcard ser/de loop:
+    //   select! {
+    //       bytes = cdc_read() => deserialize Command → commands.send()
+    //       resp  = responses.receive() => serialize Response → cdc_write()
+    //   }
+
+    loop {
+        embassy_time::Timer::after_secs(1).await;
+    }
+}