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/display/mod.rs

@@ -0,0 +1,3 @@
+mod render;
+mod task;
+pub use task::display_task;

src/display/render.rs

@@ -0,0 +1,25 @@
+use crate::channel::{RadioState, RadioStatus};
+
+/// Render the status dashboard to a framebuffer.
+///
+/// Pure logic — no hardware dependency. Takes a status snapshot and produces
+/// draw calls that the display task flushes to the screen.
+pub fn dashboard(status: &RadioStatus) {
+    // TODO: use embedded-graphics to render:
+    //   Line 1: board name + radio state
+    //   Line 2: freq / BW / SF
+    //   Line 3: TX power / coding rate
+    //   Line 4: RX count / TX count
+    //   Line 5: last RSSI / SNR
+    let _ = status;
+}
+
+/// Format radio state as a short display string.
+#[allow(dead_code)]
+pub fn state_label(state: RadioState) -> &'static str {
+    match state {
+        RadioState::Idle => "IDLE",
+        RadioState::Receiving => "RX",
+        RadioState::Transmitting => "TX",
+    }
+}

src/display/task.rs

@@ -0,0 +1,22 @@
+use embassy_executor::task;
+
+use crate::board::DisplayParts;
+use crate::channel::StatusWatch;
+
+use super::render;
+
+#[task]
+pub async fn display_task(parts: DisplayParts, status: &'static StatusWatch) {
+    let _i2c = parts.i2c;
+
+    // TODO: init SSD1306 driver from I2C handle
+    // TODO: initial render of empty dashboard
+
+    let mut receiver = status.receiver().unwrap();
+
+    loop {
+        let state = receiver.changed().await;
+        render::dashboard(&state);
+        // TODO: flush framebuffer to display
+    }
+}