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")
+    }
+}

src/board/mod.rs.j2

@@ -0,0 +1,9 @@
+// @generated by build.rs — DO NOT EDIT
+cfg_if::cfg_if! {
+    {%- for board in boards %}
+    {% if loop.first %}if{% else %}else if{% endif %} #[cfg(feature = "{{ board }}")] {
+        mod {{ board }};
+        pub use {{ board }}::*;
+    }
+    {%- endfor %}
+}

src/board/rak_4631.rs

@@ -0,0 +1,117 @@
+use embassy_embedded_hal::shared_bus::asynch::spi::SpiDevice;
+use embassy_nrf::gpio::{AnyPin, Input, Level, Output, OutputDrive, Pull};
+use embassy_nrf::peripherals::USBD;
+use embassy_nrf::spim::{self, Spim};
+use embassy_nrf::twim::{self, Twim};
+use embassy_nrf::usb::Driver;
+use embassy_nrf::{bind_interrupts, Peripherals};
+use embassy_sync::blocking_mutex::raw::NoopRawMutex;
+use embassy_time::Delay;
+use static_cell::StaticCell;
+
+use lora_phy::iv::GenericSx126xInterfaceVariant;
+use lora_phy::sx126x::{self, Sx1262, Sx126x};
+
+bind_interrupts!(struct Irqs {
+    USBD => embassy_nrf::usb::InterruptHandler<USBD>;
+    SPIM3 => embassy_nrf::spim::InterruptHandler<embassy_nrf::peripherals::SPI3>;
+    SPIM0_SPIS0_TWIM0_TWIS0_SPI0_TWI0 => embassy_nrf::twim::InterruptHandler<embassy_nrf::peripherals::TWISPI0>;
+});
+
+/// Board name shown on the display dashboard.
+#[allow(dead_code)]
+pub const BOARD_NAME: &str = "RAK 4631";
+
+// ── Concrete peripheral types ────────────────────────────────────────
+
+type SpiBus = Spim<'static, embassy_nrf::peripherals::SPI3>;
+type Nss = Output<'static>;
+type Iv = GenericSx126xInterfaceVariant<Output<'static>, Input<'static>>;
+type RadioSpiDevice = SpiDevice<'static, NoopRawMutex, SpiBus, Nss>;
+/// SX126x driver (pre-LoRa construction — LoRa::new is async).
+pub type RadioDriver = Sx126x<RadioSpiDevice, Iv, Sx1262>;
+
+/// USB driver for CDC-ACM.
+pub type UsbDriver = Driver<'static, USBD, &'static embassy_nrf::usb::vbus_detect::SoftwareVbusDetect>;
+
+/// I2C bus for an optional SSD1306 OLED.
+pub type DisplayI2c = Twim<'static, embassy_nrf::peripherals::TWISPI0>;
+
+// ── Peripheral bundles ───────────────────────────────────────────────
+
+pub struct RadioParts {
+    pub driver: RadioDriver,
+    pub delay: Delay,
+}
+
+pub struct UsbParts {
+    pub driver: UsbDriver,
+}
+
+pub struct DisplayParts {
+    pub i2c: DisplayI2c,
+}
+
+// ── Board init ───────────────────────────────────────────────────────
+
+pub struct Board {
+    p: Peripherals,
+}
+
+impl Board {
+    pub fn init() -> Self {
+        let p = embassy_nrf::init(Default::default());
+        Self { p }
+    }
+
+    pub fn into_parts(self) -> (RadioParts, UsbParts, Option<DisplayParts>) {
+        let p = self.p;
+
+        // ── SPI bus for SX1262 ───────────────────────────────────
+        let mut spi_cfg = spim::Config::default();
+        spi_cfg.frequency = spim::Frequency::M1;
+        let spi = Spim::new(p.SPI3, Irqs, p.P1_11, p.P1_13, p.P1_12, spi_cfg);
+
+        static SPI_BUS: StaticCell<embassy_sync::mutex::Mutex<NoopRawMutex, SpiBus>> =
+            StaticCell::new();
+        let spi_bus = SPI_BUS.init(embassy_sync::mutex::Mutex::new(spi));
+
+        let nss = Output::new(AnyPin::from(p.P1_10), Level::High, OutputDrive::Standard);
+        let spi_device = SpiDevice::new(spi_bus, nss);
+
+        // ── SX1262 control pins ──────────────────────────────────
+        let reset = Output::new(AnyPin::from(p.P1_06), Level::High, OutputDrive::Standard);
+        let dio1 = Input::new(AnyPin::from(p.P1_15), Pull::Down);
+        let busy = Input::new(AnyPin::from(p.P1_14), Pull::Down);
+
+        let iv = GenericSx126xInterfaceVariant::new(reset, dio1, busy, None, None).unwrap();
+
+        let sx_config = sx126x::Config {
+            chip: Sx1262,
+            tcxo_ctrl: Some(sx126x::TcxoCtrlVoltage::Ctrl1V7),
+            use_dcdc: true,
+            rx_boost: false,
+        };
+
+        let radio = RadioParts {
+            driver: Sx126x::new(spi_device, iv, sx_config),
+            delay: Delay,
+        };
+
+        // ── USB ──────────────────────────────────────────────────
+        static VBUS: StaticCell<embassy_nrf::usb::vbus_detect::SoftwareVbusDetect> =
+            StaticCell::new();
+        let vbus = VBUS.init(embassy_nrf::usb::vbus_detect::SoftwareVbusDetect::new(true, false));
+        let usb = UsbParts {
+            driver: Driver::new(p.USBD, Irqs, vbus),
+        };
+
+        // ── Display (optional RAK1921 SSD1306 OLED on I2C) ──────
+        // TODO: probe I2C address 0x3C to detect presence
+        let i2c_cfg = twim::Config::default();
+        let i2c = Twim::new(p.TWISPI0, Irqs, p.P0_13, p.P0_14, i2c_cfg);
+        let display = Some(DisplayParts { i2c });
+
+        (radio, usb, display)
+    }
+}