ws2812: add PIO support for RP2040/RP2350

Integrate PIO support directly into the existing Device.
NewWS2812() now uses PIO for hardware-timed control
on RP2040/RP2350 and falls back to bit-banging on other platforms.

No changes to the exported API surface.

ws2812/ws2812_rp2_pio.go

@@ -0,0 +1,33 @@
+//go:build rp2040 || rp2350
+
+package ws2812
+
+import (
+	"image/color"
+	"machine"
+
+	pio "github.com/tinygo-org/pio/rp2-pio"
+	"github.com/tinygo-org/pio/rp2-pio/piolib"
+)
+
+// newWS2812Device creates a WS2812 device using PIO for hardware-timed control.
+// If PIO initialization fails, it falls back to the bit-bang driver.
+func newWS2812Device(pin machine.Pin) Device {
+	sm, err := pio.PIO0.ClaimStateMachine()
+	if err != nil {
+		return Device{Pin: pin, writeColorFunc: writeColorsRGB}
+	}
+	ws, err := piolib.NewWS2812B(sm, pin)
+	if err != nil {
+		return Device{Pin: pin, writeColorFunc: writeColorsRGB}
+	}
+	return Device{
+		Pin: pin,
+		writeColorFunc: func(_ Device, buf []color.RGBA) error {
+			for _, c := range buf {
+				ws.PutRGB(c.R, c.G, c.B)
+			}
+			return nil
+		},
+	}
+}