Add modular SPI-based serial LED driver that uses SPI MOSI to generate
WS2812 timing waveforms. Uses 8-bit encoding at 6.4MHz SPI clock where
each WS2812 bit becomes one SPI byte.
SerialLED_SPI is a standalone class that can be composed into any
RCOutput implementation. RCOutput_RPI provides the RCOutput wrapper
for Raspberry Pi boards.
Features:
- Supports MODE_NEOPIXEL (GRB) and MODE_NEOPIXELRGB (RGB) color ordering
- Up to 128 LEDs per channel
- Uses /dev/spidev0.0 (GPIO10/pin 19) on Raspberry Pi
- Thread-safe with mutex protection
- Embedded object instead of heap allocation for SerialLED_SPI
- Uses calloc/free for dynamic arrays (less overhead than new/delete)
