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/channel.rs

@@ -14,7 +14,6 @@ use crate::protocol::{Command, RadioConfig, Response};
 
 /// Display commands routed from host_task to display_task.
 #[derive(Debug, Clone, Copy, defmt::Format)]
-#[allow(dead_code)] // Reset is only used by USB boards (DTR disconnect detection)
 pub enum DisplayCommand {
     On,
     Off,