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/board/heltec_v3.rs

@@ -0,0 +1,70 @@
+use esp_hal::gpio::{GpioPin, Input, Output, PullDown, PushPull};
+use esp_hal::peripherals::SPI2;
+use esp_hal::spi::master::Spi;
+use esp_hal::spi::SpiMode;
+use esp_hal::i2c::I2c;
+use esp_hal::peripherals::I2C0;
+use esp_hal::Delay;
+
+use lora_phy::iv::GenericSx126xInterfaceVariant;
+use lora_phy::sx126x::{self, Sx1262, Sx126x};
+use lora_phy::LoRa;
+
+/// Board name shown on the display dashboard.
+pub const BOARD_NAME: &str = "Heltec V3";
+
+// ── Concrete peripheral types ────────────────────────────────────────
+
+type Iv = GenericSx126xInterfaceVariant<Output<'static, GpioPin<8>>, Input<'static, GpioPin<14>>>;
+type RadioSpi = Spi<'static, SPI2>;
+type RadioDriver = Sx126x<RadioSpi, Iv, Sx1262>;
+
+/// Fully-constructed LoRa radio driver.
+pub type Radio = LoRa<RadioDriver, Delay>;
+
+/// USB driver for CDC-ACM.
+// TODO: define concrete USB OTG type once esp-hal USB support is wired up
+pub type UsbDriver = ();
+
+/// I2C bus for the built-in SSD1306 OLED.
+pub type DisplayI2c = I2c<'static, I2C0>;
+
+// ── Peripheral bundles ───────────────────────────────────────────────
+
+pub struct RadioParts {
+    pub radio: Radio,
+}
+
+pub struct UsbParts {
+    pub driver: UsbDriver,
+}
+
+pub struct DisplayParts {
+    pub i2c: DisplayI2c,
+}
+
+// ── Board init ───────────────────────────────────────────────────────
+
+pub struct Board {
+    p: esp_hal::peripherals::Peripherals,
+}
+
+impl Board {
+    pub fn init() -> Self {
+        let p = esp_hal::init(esp_hal::Config::default());
+        Self { p }
+    }
+
+    pub fn into_parts(self) -> (RadioParts, UsbParts, Option<DisplayParts>) {
+        let _p = self.p;
+
+        // TODO: implement Heltec V3 peripheral init
+        // - SPI2 for SX1262 (SCK=9, MOSI=10, MISO=11, NSS=8)
+        // - GPIO for radio control (RST=12, DIO1=14, BUSY=13)
+        // - Vext pin (GPIO21) LOW to power OLED
+        // - I2C0 for SSD1306 (SDA=17, SCL=18)
+        // - USB OTG for CDC-ACM
+
+        todo!("Heltec V3 peripheral init — requires espup toolchain to develop")
+    }
+}