feat: add Heltec V4, wire up V3/V4 peripherals, restructure entry point

- Add heltec_v4 board: ESP32-S3R2, SX1262, SSD1315 display, native USB,
  Vext on GPIO36 (active HIGH)
- Wire up heltec_v3 with real esp-hal peripherals (was a stub)
- V3 and V4 share radio/I2C pins, differ in Vext pin/polarity and
  display reset
- Restructure main.rs: cfg_if selects embassy_executor::main (Cortex-M)
  vs esp_hal_embassy::main (Xtensa) per board
- Display detection via SSD1306 init in display_task — boards always
  provide I2C bus, task exits gracefully if no display responds

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Justfile

@@ -4,12 +4,13 @@ set shell := ["bash", "-c"]
 # Xtensa targets use nightly cargo + esp rustc + -Zbuild-std=core.
 # check-all gracefully skips boards whose toolchain isn't installed.
 heltec_v3 := "heltec_v3 xtensa-esp32s3-none-elf esp32s3"
+heltec_v4 := "heltec_v4 xtensa-esp32s3-none-elf esp32s3"
 rak_4631  := "rak_4631 thumbv7em-none-eabihf nRF52840_xxAA"
 
 firmware_dir := "firmware"
 
 # All known boards
-boards := "heltec_v3 rak_4631"
+boards := "heltec_v3 heltec_v4 rak_4631"
 
 # Check all boards that can build with the available toolchain
 check-all:
@@ -79,5 +80,6 @@ _copy_firmware board profile:
 [private]
 _info name:
     @if [ "{{name}}" == "heltec_v3" ]; then echo "{{heltec_v3}}"; \
+     elif [ "{{name}}" == "heltec_v4" ]; then echo "{{heltec_v4}}"; \
      elif [ "{{name}}" == "rak_4631" ]; then echo "{{rak_4631}}"; \
      else echo "Unknown board: {{name}}" >&2; exit 1; fi