fix: V3 has white LED on GPIO35, not WS2812B on GPIO38

The Heltec V3 has a plain white LED on GPIO35 (simple GPIO, not
addressable). Only the V4 has a WS2812B RGB LED on GPIO38.

- Created driver/simple_led.rs for single-color GPIO LEDs
- V3/V3-UART boards: SimpleLed on GPIO35 (white blink on RX/TX)
- V4 board: unchanged (WS2812 on GPIO38, green/red colors)
- nRF boards: unchanged (no LED, type LedDriver = ())

src/driver/mod.rs

@@ -1,5 +1,8 @@
 #[cfg(feature = "wio_tracker_l1")]
 pub mod sh1106;
 
-#[cfg(any(feature = "heltec_v3", feature = "heltec_v3_uart", feature = "heltec_v4"))]
+#[cfg(any(feature = "heltec_v3", feature = "heltec_v3_uart"))]
+pub mod simple_led;
+
+#[cfg(feature = "heltec_v4")]
 pub mod ws2812;

src/driver/simple_led.rs

@@ -0,0 +1,28 @@
+//! Simple single-color LED driver (GPIO on/off).
+//!
+//! For boards with a plain LED (not addressable RGB). Any non-zero
+//! RGB value turns the LED on; all zeros turns it off.
+
+use esp_hal::gpio::Output;
+
+use crate::board::RgbLed;
+
+pub struct SimpleLed {
+    pin: Output<'static>,
+}
+
+impl SimpleLed {
+    pub fn new(pin: Output<'static>) -> Self {
+        Self { pin }
+    }
+}
+
+impl RgbLed for SimpleLed {
+    async fn set_rgb(&mut self, r: u8, g: u8, b: u8) {
+        if r > 0 || g > 0 || b > 0 {
+            self.pin.set_high();
+        } else {
+            self.pin.set_low();
+        }
+    }
+}