feat: native USB CDC-ACM support for Heltec V3

The default heltec_v3 feature now uses native USB OTG (same as V4),
appearing as /dev/ttyACM0 with the DongLoRa VID:PID (1209:5741).
This requires a hardware mod: solder R29/R3 and disconnect the CP2102
from the USB data lines.

Stock (unmodified) V3 boards should use the new heltec_v3_uart feature
which preserves the original UART-over-CP2102 behavior (/dev/ttyUSB0).

- heltec_v3: switched from UART to USB OTG
- heltec_v3_uart: new feature, UART fallback for stock boards
- Removed #[cfg] gates on UsbParts/UartParts (both always defined)
- Updated README with hardware mod instructions and board table
- Updated justfile with heltec_v3_uart board definition

firmware/justfile

@@ -3,8 +3,9 @@ set shell := ["bash", "-c"]
 # Board definitions: feature target chip
 # Xtensa targets use nightly cargo + esp rustc + -Zbuild-std=core.
 # Tool versions are pinned in mise.toml; run `just setup` to install everything.
-heltec_v3 := "heltec_v3 xtensa-esp32s3-none-elf esp32s3"
-heltec_v4 := "heltec_v4 xtensa-esp32s3-none-elf esp32s3"
+heltec_v3      := "heltec_v3 xtensa-esp32s3-none-elf esp32s3"
+heltec_v3_uart := "heltec_v3_uart xtensa-esp32s3-none-elf esp32s3"
+heltec_v4      := "heltec_v4 xtensa-esp32s3-none-elf esp32s3"
 rak_wisblock_4631  := "rak_wisblock_4631 thumbv7em-none-eabihf nRF52840_xxAA"
 wio_tracker_l1     := "wio_tracker_l1 thumbv7em-none-eabihf nRF52840_xxAA"
 
@@ -12,7 +13,7 @@ builds_dir := "builds"
 version := `sed -n 's/^version = "\(.*\)"/\1/p' Cargo.toml`
 
 # All known boards
-boards := "heltec_v3 heltec_v4 rak_wisblock_4631 wio_tracker_l1"
+boards := "heltec_v3 heltec_v3_uart heltec_v4 rak_wisblock_4631 wio_tracker_l1"
 
 # Install all required tools and toolchains
 setup:
@@ -203,6 +204,7 @@ _flash_uf2 board:
 [private]
 _info name:
     @if [ "{{name}}" == "heltec_v3" ]; then echo "{{heltec_v3}}"; \
+     elif [ "{{name}}" == "heltec_v3_uart" ]; then echo "{{heltec_v3_uart}}"; \
      elif [ "{{name}}" == "heltec_v4" ]; then echo "{{heltec_v4}}"; \
      elif [ "{{name}}" == "rak_wisblock_4631" ]; then echo "{{rak_wisblock_4631}}"; \
      elif [ "{{name}}" == "wio_tracker_l1" ]; then echo "{{wio_tracker_l1}}"; \