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/board/heltec_v4.rs

@@ -1,8 +1,11 @@
 use esp_hal::gpio::{Level, Output, OutputConfig};
+use esp_hal::rmt::{Rmt, TxChannelConfig, TxChannelCreator};
+use esp_hal::time::Rate;
 use static_cell::StaticCell;
 
 use super::esp32s3;
 use super::traits::{BoardParts, LoRaBoard};
+use crate::driver::ws2812::Ws2812;
 use crate::hal::esp32s3 as mcu;
 
 pub use super::esp32s3::{
@@ -11,6 +14,8 @@ pub use super::esp32s3::{
 
 pub type UsbDriver = esp_hal::otg_fs::asynch::Driver<'static>;
 
+pub type LedDriver = Ws2812;
+
 pub struct UsbParts {
     pub driver: UsbDriver,
 }
@@ -27,6 +32,7 @@ impl LoRaBoard for Board {
     type CommParts = UsbParts;
     type DisplayParts = DisplayParts;
     type DisplayDriver = DisplayDriver;
+    type LedDriver = Ws2812;
 
     fn init() -> Self {
         let p = esp_hal::init(esp_hal::Config::default());
@@ -37,7 +43,7 @@ impl LoRaBoard for Board {
         mcu::mac_address()
     }
 
-    fn into_parts(self) -> BoardParts<RadioParts, UsbParts, DisplayParts> {
+    fn into_parts(self) -> BoardParts<RadioParts, UsbParts, DisplayParts, Ws2812> {
         let p = self.p;
 
         mcu::start_timer(p.TIMG0);
@@ -71,10 +77,25 @@ impl LoRaBoard for Board {
         let i2c = mcu::init_i2c(p.I2C0, p.GPIO17, p.GPIO18);
         let display = Some(DisplayParts { i2c });
 
+        // WS2812B RGB LED on GPIO38 via RMT channel 0
+        let rmt = Rmt::new(p.RMT, Rate::from_mhz(80))
+            .expect("RMT init")
+            .into_async();
+        let led_channel = rmt.channel0
+            .configure_tx(
+                p.GPIO38,
+                TxChannelConfig::default()
+                    .with_clk_divider(1)
+                    .with_idle_output(true),
+            )
+            .expect("RMT TX channel init");
+        let led = Some(Ws2812::new(led_channel));
+
         BoardParts {
             radio,
             host,
             display,
+            led,
             mac: Self::mac_address(),
         }
     }