refactor: drop Heltec V3, enforce standard toolchain, remove dead code

Heltec V3 (ESP32-S3) requires the espup Xtensa toolchain fork, which
violates the standard-toolchain-only principle. Removed until upstream
Rust supports Xtensa targets via stock rustup.

Also removes all #[allow(dead_code)] band-aids — unused items are
deleted rather than suppressed. DESIGN.md updated with standard
toolchain principle (#10) and peripheral model (input/telemetry slots).

src/board/heltec_v3.rs

@@ -1,70 +0,0 @@
-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/rak_4631.rs

@@ -18,10 +18,6 @@ bind_interrupts!(struct Irqs {
     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>;

src/display/render.rs

@@ -1,4 +1,4 @@
-use crate::channel::{RadioState, RadioStatus};
+use crate::channel::RadioStatus;
 
 /// Render the status dashboard to a framebuffer.
 ///
@@ -13,13 +13,3 @@ pub fn dashboard(status: &RadioStatus) {
     //   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/error.rs

@@ -1,16 +0,0 @@
-/// Radio hardware errors.
-#[allow(dead_code)]
-#[derive(Debug, defmt::Format)]
-pub enum RadioError {
-    InitFailed,
-    TxTimeout,
-    InvalidConfig,
-}
-
-/// USB protocol framing errors.
-#[allow(dead_code)]
-#[derive(Debug, defmt::Format)]
-pub enum ProtocolError {
-    DeserializeFailed,
-    BufferOverflow,
-}

src/main.rs

@@ -4,7 +4,6 @@
 mod board;
 mod channel;
 mod display;
-mod error;
 mod protocol;
 mod radio;
 mod usb;
@@ -16,8 +15,6 @@ use crate::channel::{CommandChannel, ResponseChannel, StatusWatch};
 cfg_if::cfg_if! {
     if #[cfg(feature = "rak_4631")] {
         use panic_probe as _;
-    } else if #[cfg(feature = "heltec_v3")] {
-        use esp_backtrace as _;
     }
 }
 

src/protocol.rs

@@ -4,10 +4,6 @@ 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)]