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/protocol.rs

@@ -277,12 +277,11 @@ impl Response {
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 #[cfg_attr(not(test), derive(defmt::Format))]
 #[repr(u8)]
-#[allow(dead_code)] // CrcError reserved per PROTOCOL.md (error code 3)
 pub enum ErrorCode {
     InvalidConfig = 0,
     RadioBusy = 1,
     TxTimeout = 2,
-    CrcError = 3,
+    // 3 reserved (was CrcError — SX1262 silently drops bad-CRC packets)
     NotConfigured = 4,
     NoDisplay = 5,
 }
@@ -686,7 +685,6 @@ mod tests {
             (ErrorCode::InvalidConfig, 0),
             (ErrorCode::RadioBusy, 1),
             (ErrorCode::TxTimeout, 2),
-            (ErrorCode::CrcError, 3),
             (ErrorCode::NotConfigured, 4),
             (ErrorCode::NoDisplay, 5),
         ] {