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

firmware/TODO.md

@@ -1,8 +1,4 @@
-- get the code better organized
-- firmware file naming, no "release", add version
 - LED blinking on TX/RX? colored?
-- display having board specific stuff in it
 - publish new client, mux, bridge (updates in client for UART crap)
-- try to get heltec v3 back to the right USB port+VID:PID
 - split out mux and client repos
 - move to GH organization