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;