neopixel: add PIO-based WS2812B driver for RP2040/RP2350

[gandarez]

Summary

• Adds a new neopixel package that provides an Adafruit NeoPixel-like API for WS2812B RGB LED strips
• Uses PIO (via tinygo-org/pio) for hardware-timed control on RP2040/RP2350, fixing the issue where the existing ws2812 bit-bang driver fails on RP2350's Cortex-M33
• Falls back to the existing ws2812 bit-bang driver on all other platforms
• Includes an example targeting the Waveshare RP2350-LCD-1.47 board (GP22, 1 LED)

Motivation

The existing ws2812 driver uses CPU-timed bit-banging assembly tuned for Cortex-M0+. The RP2350's Cortex-M33 has a write buffer and different pipeline that breaks the nanosecond-level GPIO timing. The Arduino Adafruit_NeoPixel library works on the same hardware because it uses PIO.

API

pixel, err := neopixel.New(machine.GP22, 1)
pixel.SetBrightness(50)
pixel.SetPixel(0, color.RGBA{R: 255})
pixel.Show()
pixel.Fill(color.RGBA{G: 255})
pixel.Clear()

Test plan

• tinygo flash -target=pico2 ./examples/neopixel/ — verify LED cycles Red → Green → Blue on Waveshare RP2350
• Verify brightness control matches Arduino behavior at brightness=50
• Verify fallback compiles on non-RP targets

🤖 Generated with Claude Code

[soypat]

Looks good! Ready to merge?

[deadprogram]

@gandarez I just wonder if this might be able to be added directly to the existing ws2812 driver instead of adding another driver. I have not looked but seems like with the correct build tags you could bypass the CPU and use the PIO code in this PR instead.

What do you think?

[gandarez]

Let me check if it's possible

[gandarez]

Closing due to #851