fix: switch WS2812 driver to blocking RMT transmit

Async RMT had peripheral lifetime issues causing silent failures.
Blocking transmit is ~31µs per update (negligible) and avoids the
async complexity. Also removes the ManuallyDrop/unsafe workaround.

firmware/src/board/heltec_v4.rs

@@ -77,18 +77,10 @@ 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.
-        // Wrap in ManuallyDrop so the Rmt peripheral isn't disabled when
-        // the struct is dropped after we take channel0.
-        let rmt = core::mem::ManuallyDrop::new(
-            Rmt::new(p.RMT, Rate::from_mhz(80))
-                .expect("RMT init")
-                .into_async(),
-        );
-        // SAFETY: we're reading channel0 out of the ManuallyDrop wrapper.
-        // The Rmt won't be dropped, so the peripheral stays enabled.
-        let channel0 = unsafe { core::ptr::read(&raw const rmt.channel0) };
-        let led_channel = channel0
+        // WS2812B RGB LED on GPIO38 via RMT channel 0 (blocking, ~31µs per update)
+        let rmt = Rmt::new(p.RMT, Rate::from_mhz(80)).expect("RMT init");
+        let led_channel = rmt
+            .channel0
             .configure_tx(
                 p.GPIO38,
                 TxChannelConfig::default()

firmware/src/driver/ws2812.rs

@@ -2,6 +2,7 @@
 //!
 //! Encodes RGB values as GRB PulseCodes and transmits via RMT.
 //! At 80 MHz with divider=1, each tick is 12.5 ns.
+//! Uses blocking transmit (~31µs per LED update).
 
 use esp_hal::gpio::Level;
 use esp_hal::rmt::{Channel, PulseCode, Tx};
@@ -14,14 +15,16 @@ 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.
+/// WS2812B single-LED driver over RMT (blocking).
 pub struct Ws2812 {
-    channel: Channel<'static, esp_hal::Async, Tx>,
+    channel: Option<Channel<'static, esp_hal::Blocking, Tx>>,
 }
 
 impl Ws2812 {
-    pub fn new(channel: Channel<'static, esp_hal::Async, Tx>) -> Self {
-        Self { channel }
+    pub fn new(channel: Channel<'static, esp_hal::Blocking, Tx>) -> Self {
+        Self {
+            channel: Some(channel),
+        }
     }
 
     /// Encode 3 bytes (GRB order) + end marker into PulseCodes.
@@ -35,7 +38,6 @@ impl Ws2812 {
                 idx += 1;
             }
         }
-        // data[24] is already end_marker
         data
     }
 }
@@ -43,8 +45,20 @@ impl Ws2812 {
 impl RgbLed for Ws2812 {
     async fn set_rgb(&mut self, r: u8, g: u8, b: u8) {
         let data = Self::encode(r, g, b);
-        if let Err(e) = self.channel.transmit(&data).await {
-            defmt::warn!("WS2812 transmit error: {}", e);
+        if let Some(ch) = self.channel.take() {
+            match ch.transmit(&data) {
+                Ok(tx) => match tx.wait() {
+                    Ok(ch) => self.channel = Some(ch),
+                    Err((e, ch)) => {
+                        defmt::warn!("WS2812 wait error: {}", e);
+                        self.channel = Some(ch);
+                    }
+                },
+                Err(e) => {
+                    defmt::warn!("WS2812 transmit error: {}", e);
+                    // Channel consumed on transmit error — LED is lost
+                }
+            }
         }
     }
 }