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/host/uart.rs

@@ -6,7 +6,7 @@ use defmt::warn;
 use embassy_executor::task;
 use embedded_io_async::Write;
 
-use crate::channel::{CommandChannel, DisplayCommandChannel, ResponseChannel};
+use crate::channel::{CommandChannel, DisplayCommand, DisplayCommandChannel, ResponseChannel};
 use super::framing::{self, CobsDecoder, MAX_FRAME};
 
 #[task]
@@ -20,6 +20,11 @@ pub async fn host_task(
 ) {
     let (mut rx, mut tx) = parts.driver.split();
 
+    // UART has no DTR — signal the display that a host is present at boot.
+    if has_display {
+        display_commands.send(DisplayCommand::Reset).await;
+    }
+
     let mut read_buf = [0u8; 64];
     let mut write_buf = [0u8; MAX_FRAME];
     let mut cobs_encode_buf = [0u8; MAX_FRAME];