feat: RGB LED support — blinks green on RX, red on TX

- Added RgbLed trait with impl on () for boards without LED
- Added WS2812 driver (driver/ws2812.rs) using ESP32-S3 RMT peripheral
  - 80MHz clock, 12.5ns resolution, 24-bit GRB encoding
  - Zero extra crate dependencies
- Heltec V3/V3-UART/V4: WS2812B on GPIO38 via RMT channel 0
- RAK WisBlock/Wio Tracker: type LedDriver = (), led = None (no-op)
- Display task: green blink on RX packet, red blink on TX packet
- LED auto-off on timer tick (~1s), off when display is off/reset
- LoRaBoard trait gains type LedDriver: RgbLed associated type
- BoardParts gains led: Option<L> field

src/driver/mod.rs

@@ -1,2 +1,5 @@
 #[cfg(feature = "wio_tracker_l1")]
 pub mod sh1106;
+
+#[cfg(any(feature = "heltec_v3", feature = "heltec_v3_uart", feature = "heltec_v4"))]
+pub mod ws2812;

src/driver/ws2812.rs

@@ -0,0 +1,48 @@
+//! WS2812B single-LED driver using the ESP32-S3 RMT peripheral.
+//!
+//! Encodes RGB values as GRB PulseCodes and transmits via RMT.
+//! At 80 MHz with divider=1, each tick is 12.5 ns.
+
+use esp_hal::gpio::Level;
+use esp_hal::rmt::{Channel, PulseCode, Tx};
+
+use crate::board::RgbLed;
+
+/// WS2812 0-bit: High 400ns (32 ticks), Low 850ns (68 ticks).
+const BIT_0: PulseCode = PulseCode::new(Level::High, 32, Level::Low, 68);
+
+/// WS2812 1-bit: High 800ns (64 ticks), Low 450ns (36 ticks).
+const BIT_1: PulseCode = PulseCode::new(Level::High, 64, Level::Low, 36);
+
+/// WS2812B single-LED driver over RMT.
+pub struct Ws2812 {
+    channel: Channel<'static, esp_hal::Async, Tx>,
+}
+
+impl Ws2812 {
+    pub fn new(channel: Channel<'static, esp_hal::Async, Tx>) -> Self {
+        Self { channel }
+    }
+
+    /// Encode 3 bytes (GRB order) + end marker into PulseCodes.
+    fn encode(r: u8, g: u8, b: u8) -> [PulseCode; 25] {
+        let mut data = [PulseCode::end_marker(); 25];
+        let bytes = [g, r, b]; // WS2812 expects GRB order
+        let mut idx = 0;
+        for byte in bytes {
+            for bit in (0..8).rev() {
+                data[idx] = if byte & (1 << bit) != 0 { BIT_1 } else { BIT_0 };
+                idx += 1;
+            }
+        }
+        // data[24] is already end_marker
+        data
+    }
+}
+
+impl RgbLed for Ws2812 {
+    async fn set_rgb(&mut self, r: u8, g: u8, b: u8) {
+        let data = Self::encode(r, g, b);
+        let _ = self.channel.transmit(&data).await;
+    }
+}