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>;