feat(firmware): blink on-board WS2812 alongside GPIO 2

The DevKitC-1 has no discrete LED on GPIO 2, so the Spike 1 blink was
invisible without external wiring. Drive the on-board addressable LED
(WS2812 on GPIO 48) each cycle for visible confirmation on the bench.

firmware/src/main.rs

@@ -1,6 +1,14 @@
+use std::time::Duration;
+
 use esp_idf_svc::hal::delay::FreeRtos;
 use esp_idf_svc::hal::gpio::PinDriver;
 use esp_idf_svc::hal::peripherals::Peripherals;
+use esp_idf_svc::hal::rmt::config::{TransmitConfig, TxChannelConfig};
+use esp_idf_svc::hal::rmt::encoder::CopyEncoder;
+use esp_idf_svc::hal::rmt::{PinState, Symbol, TxChannelDriver};
+use esp_idf_svc::hal::units::Hertz;
+
+const WS2812_RESOLUTION: Hertz = Hertz(10_000_000);
 
 fn main() -> anyhow::Result<()> {
     // Required once before any esp-idf-svc call; some runtime patches
@@ -11,16 +19,59 @@ fn main() -> anyhow::Result<()> {
     let peripherals = Peripherals::take()?;
     let mut led = PinDriver::output(peripherals.pins.gpio2)?;
 
-    log::info!("Typoena Spike 1 — Blink on GPIO 2");
+    // On-board addressable LED (WS2812) — GPIO 48 on the DevKitC-1 v1.0;
+    // v1.1 boards moved it to GPIO 38.
+    let mut rgb = TxChannelDriver::new(
+        peripherals.pins.gpio48,
+        &TxChannelConfig {
+            resolution: WS2812_RESOLUTION,
+            ..Default::default()
+        },
+    )?;
+
+    log::info!("Typoena Spike 1 — Blink on GPIO 2 + on-board WS2812 (GPIO 48)");
 
     let mut n: u32 = 0;
     loop {
         led.set_high()?;
+        ws2812_set(&mut rgb, 4, 0, 24)?; // dim green
         log::info!("blink {n}");
         n = n.wrapping_add(1);
         FreeRtos::delay_ms(500);
 
         led.set_low()?;
+        ws2812_set(&mut rgb, 0, 0, 0)?;
         FreeRtos::delay_ms(500);
     }
 }
+
+/// Send one WS2812 GRB frame. The ≥50 µs low reset the LED needs between
+/// frames is covered by the 500 ms idle gap between calls.
+fn ws2812_set(tx: &mut TxChannelDriver, r: u8, g: u8, b: u8) -> anyhow::Result<()> {
+    // Bit timings from the WS2812 datasheet.
+    let zero = Symbol::new_with(
+        WS2812_RESOLUTION,
+        PinState::High,
+        Duration::from_nanos(350),
+        PinState::Low,
+        Duration::from_nanos(800),
+    )?;
+    let one = Symbol::new_with(
+        WS2812_RESOLUTION,
+        PinState::High,
+        Duration::from_nanos(700),
+        PinState::Low,
+        Duration::from_nanos(600),
+    )?;
+
+    let mut symbols = [zero; 24];
+    for (i, byte) in [g, r, b].into_iter().enumerate() {
+        for bit in 0..8 {
+            if byte & (0x80 >> bit) != 0 {
+                symbols[i * 8 + bit] = one;
+            }
+        }
+    }
+    tx.send_and_wait(CopyEncoder::new()?, &symbols, &TransmitConfig::default())?;
+    Ok(())
+}