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/channel.rs

@@ -0,0 +1,46 @@
+use embassy_sync::blocking_mutex::raw::CriticalSectionRawMutex;
+use embassy_sync::channel::Channel;
+use embassy_sync::watch::Watch;
+
+use crate::protocol::{Command, RadioConfig, Response};
+
+/// Host-to-radio command channel.
+pub type CommandChannel = Channel<CriticalSectionRawMutex, Command, 16>;
+
+/// Radio-to-host response channel.
+pub type ResponseChannel = Channel<CriticalSectionRawMutex, Response, 32>;
+
+/// Observable radio status for the display task.
+pub type StatusWatch = Watch<CriticalSectionRawMutex, RadioStatus, 2>;
+
+/// Current radio state exposed to observers (e.g. display).
+#[derive(Debug, Clone, defmt::Format)]
+pub struct RadioStatus {
+    pub state: RadioState,
+    pub config: Option<RadioConfig>,
+    pub rx_count: u32,
+    pub tx_count: u32,
+    pub last_rssi: Option<i16>,
+    pub last_snr: Option<i8>,
+}
+
+/// Radio state machine states.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, defmt::Format)]
+pub enum RadioState {
+    Idle,
+    Receiving,
+    Transmitting,
+}
+
+impl Default for RadioStatus {
+    fn default() -> Self {
+        Self {
+            state: RadioState::Idle,
+            config: None,
+            rx_count: 0,
+            tx_count: 0,
+            last_rssi: None,
+            last_snr: None,
+        }
+    }
+}