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/Cargo.toml

@@ -84,6 +84,12 @@ heltec_v3 = [
   "dep:esp-println",
   "dep:esp-rtos",
 ]
+heltec_v3_uart = [
+  "dep:esp-backtrace",
+  "dep:esp-hal",
+  "dep:esp-println",
+  "dep:esp-rtos",
+]
 heltec_v4 = [
   "dep:esp-backtrace",
   "dep:esp-hal",