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>

Cargo.toml

@@ -48,6 +48,12 @@ heltec_v3 = [
     "dep:esp-hal-embassy",
     "dep:esp-println",
 ]
+heltec_v4 = [
+    "dep:esp-backtrace",
+    "dep:esp-hal",
+    "dep:esp-hal-embassy",
+    "dep:esp-println",
+]
 rak_4631 = [
     "dep:cortex-m-rt",
     "dep:embassy-nrf",