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

justfile

@@ -57,11 +57,11 @@ check-all:
 
 # Check a single board compiles
 check board:
-    @just _cargo {{board}} check
+    @just _cargo {{board}} "check --release"
 
 # Run clippy on a single board
 clippy board:
-    @just _cargo {{board}} "clippy" "-- -D warnings"
+    @just _cargo {{board}} "clippy --release" "-- -D warnings"
 
 # Build release firmware and copy to builds/ with a readable name
 build board profile="release":