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/board/heltec_v3.rs

@@ -7,17 +7,22 @@
 //! For stock (unmodified) V3 boards, use the `heltec_v3_uart` feature instead.
 
 use esp_hal::gpio::{Level, Output, OutputConfig};
+use static_cell::StaticCell;
 
 use super::esp32s3;
 use super::traits::{BoardParts, LoRaBoard};
 use crate::hal::esp32s3 as mcu;
 
-#[allow(unused_imports)] // Re-exported for other modules (display, radio, host tasks)
 pub use super::esp32s3::{
-    create_display, DisplayDriver, DisplayI2c, DisplayParts, RadioDriver, RadioParts, UsbDriver,
-    UsbParts,
+    create_display, DisplayDriver, DisplayParts, RadioDriver, RadioParts,
 };
 
+pub type UsbDriver = esp_hal::otg_fs::asynch::Driver<'static>;
+
+pub struct UsbParts {
+    pub driver: UsbDriver,
+}
+
 pub struct Board {
     p: esp_hal::peripherals::Peripherals,
 }
@@ -53,9 +58,15 @@ impl LoRaBoard for Board {
         let radio = esp32s3::init_radio(spi_bus, p.GPIO8, p.GPIO12, p.GPIO14, p.GPIO13);
 
         // Note: switches internal USB PHY from Serial-JTAG to OTG.
-        // espflash --monitor will stop working after this point.
+        let usb_inst = esp_hal::otg_fs::Usb::new(p.USB0, p.GPIO20, p.GPIO19);
+        static EP_OUT_BUF: StaticCell<[u8; 1024]> = StaticCell::new();
+        let ep_out_buf = EP_OUT_BUF.init([0u8; 1024]);
         let host = UsbParts {
-            driver: mcu::init_usb(p.USB0, p.GPIO20, p.GPIO19),
+            driver: esp_hal::otg_fs::asynch::Driver::new(
+                usb_inst,
+                ep_out_buf,
+                esp_hal::otg_fs::asynch::Config::default(),
+            ),
         };
 
         // SSD1306 display reset: pulse GPIO21 low->high before I2C init