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/protocol.rs

@@ -0,0 +1,89 @@
+use heapless::Vec;
+use serde::{Deserialize, Serialize};
+
+/// Maximum LoRa payload size in bytes.
+pub const MAX_PAYLOAD: usize = 256;
+
+/// Protocol version for host compatibility checks.
+#[allow(dead_code)]
+pub const PROTOCOL_VERSION: u8 = 1;
+
+/// LoRa signal bandwidth.
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, defmt::Format)]
+#[repr(u8)]
+pub enum Bandwidth {
+    Khz7 = 0,
+    Khz10 = 1,
+    Khz15 = 2,
+    Khz20 = 3,
+    Khz31 = 4,
+    Khz41 = 5,
+    Khz62 = 6,
+    Khz125 = 7,
+    Khz250 = 8,
+    Khz500 = 9,
+}
+
+/// LoRa forward error correction coding rate.
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, defmt::Format)]
+#[repr(u8)]
+pub enum CodingRate {
+    Cr4_5 = 1,
+    Cr4_6 = 2,
+    Cr4_7 = 3,
+    Cr4_8 = 4,
+}
+
+/// Complete LoRa radio configuration.
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, defmt::Format)]
+pub struct RadioConfig {
+    pub freq_hz: u32,
+    pub bw: Bandwidth,
+    pub sf: u8,
+    pub cr: CodingRate,
+    pub sync_word: u16,
+    pub tx_power_dbm: i8,
+}
+
+/// Host → firmware commands.
+// Payload variant is intentionally large (inline heapless::Vec) — no allocator available.
+#[allow(clippy::large_enum_variant)]
+#[derive(Serialize, Deserialize, Debug, PartialEq)]
+pub enum Command {
+    Ping,
+    GetConfig,
+    SetConfig(RadioConfig),
+    StartRx,
+    StopRx,
+    Transmit {
+        config: Option<RadioConfig>,
+        payload: Vec<u8, MAX_PAYLOAD>,
+    },
+}
+
+/// Firmware → host responses.
+// Payload variant is intentionally large (inline heapless::Vec) — no allocator available.
+#[allow(clippy::large_enum_variant)]
+#[derive(Serialize, Deserialize, Debug, PartialEq)]
+pub enum Response {
+    Pong,
+    Config(RadioConfig),
+    RxPacket {
+        rssi: i16,
+        snr: i8,
+        payload: Vec<u8, MAX_PAYLOAD>,
+    },
+    TxDone,
+    Error(ErrorCode),
+}
+
+/// Error codes reported to the host.
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, defmt::Format)]
+#[repr(u8)]
+pub enum ErrorCode {
+    InvalidConfig = 1,
+    RadioBusy = 2,
+    TxTimeout = 3,
+    CrcError = 4,
+    NotConfigured = 5,
+}