feat: Heltec V3 native USB, remove all dead_code suppressions

- heltec_v3 now uses USB OTG (requires hardware mod: solder R29/R3)
- heltec_v3_uart added as fallback for stock boards
- Removed every #[allow(dead_code)] — fixed root causes instead:
  - Comm types (UsbParts/UartParts) moved to individual board files
  - USB/UART init removed from hal (boards call esp-hal directly)
  - SH1106 driver gated on boards that use it
  - UART task sends DisplayCommand::Reset at startup (variant now used)
  - ErrorCode::CrcError removed (SX1262 drops bad-CRC silently)
- check/clippy now use --release (no more Xtensa dev-mode warnings)
- Updated README with hardware mod docs and board table

src/hal/esp32s3.rs

@@ -1,8 +1,8 @@
 //! ESP32-S3 MCU initialization primitives.
 //!
-//! Provides low-level bus and peripheral init for ESP32-S3 boards.
+//! Provides low-level bus init for ESP32-S3 boards.
 //! Board files call these with specific pins and construct higher-level
-//! drivers (Sx126x, SSD1306, etc.) themselves.
+//! drivers (Sx126x, SSD1306, USB, UART, etc.) themselves.
 
 use embassy_sync::blocking_mutex::raw::NoopRawMutex;
 use esp_hal::dma::{DmaRxBuf, DmaTxBuf};
@@ -17,10 +17,6 @@ use static_cell::StaticCell;
 
 pub type SpiBus = SpiDmaBus<'static, esp_hal::Async>;
 pub type I2cBus = I2c<'static, esp_hal::Async>;
-#[allow(dead_code)] // Used by USB boards, not UART boards
-pub type UsbOtgDriver = esp_hal::otg_fs::asynch::Driver<'static>;
-#[allow(dead_code)] // Used by UART boards, not USB boards
-pub type UartDriver = esp_hal::uart::Uart<'static, esp_hal::Async>;
 
 // ── Timer ───────────────────────────────────────────────────────────
 
@@ -33,8 +29,6 @@ pub fn start_timer(timg0: esp_hal::peripherals::TIMG0<'static>) {
 // ── SPI bus ─────────────────────────────────────────────────────────
 
 /// Initialize SPI2 with DMA and wrap in a shared bus (StaticCell + Mutex).
-///
-/// Returns a reference to the shared SPI bus suitable for `SpiDevice::new()`.
 pub fn init_spi(
     spi2: esp_hal::peripherals::SPI2<'static>,
     dma_ch0: esp_hal::peripherals::DMA_CH0<'static>,
@@ -80,43 +74,6 @@ pub fn init_i2c(
         .into_async()
 }
 
-// ── USB OTG ─────────────────────────────────────────────────────────
-
-/// Initialize USB OTG FS driver for CDC-ACM.
-#[allow(dead_code)] // Used by USB boards, not UART boards
-pub fn init_usb(
-    usb0: esp_hal::peripherals::USB0<'static>,
-    dp: esp_hal::peripherals::GPIO20<'static>,
-    dm: esp_hal::peripherals::GPIO19<'static>,
-) -> UsbOtgDriver {
-    let usb_inst = esp_hal::otg_fs::Usb::new(usb0, dp, dm);
-    static EP_OUT_BUF: StaticCell<[u8; 1024]> = StaticCell::new();
-    let ep_out_buf = EP_OUT_BUF.init([0u8; 1024]);
-    esp_hal::otg_fs::asynch::Driver::new(
-        usb_inst,
-        ep_out_buf,
-        esp_hal::otg_fs::asynch::Config::default(),
-    )
-}
-
-// ── UART ────────────────────────────────────────────────────────────
-
-/// Initialize UART0 for boards with USB-UART bridge chips (e.g. CP2102).
-#[allow(dead_code)] // Used by UART boards, not USB boards
-pub fn init_uart(
-    uart0: esp_hal::peripherals::UART0<'static>,
-    tx: esp_hal::peripherals::GPIO43<'static>,
-    rx: esp_hal::peripherals::GPIO44<'static>,
-) -> UartDriver {
-    use esp_hal::uart::{Config as UartConfig, Uart};
-
-    Uart::new(uart0, UartConfig::default())
-        .expect("UART init")
-        .with_tx(tx)
-        .with_rx(rx)
-        .into_async()
-}
-
 // ── MAC address ─────────────────────────────────────────────────────
 
 /// Read the factory-programmed MAC address from eFuse.