Update the Pico SDK to 2.3.1

[mikeysklar]

What

Updates the Pico SDK from 2.2.0 to 2.3.1. Also fixes pinless PIO state machines on RP2350B boards (Metro RP2350, Fruit Jam).

Why

Related to #11098. SDK 2.3 intentionally rejects zero pins on RP2350B (raspberrypi/pico-sdk#2989); pinless callers now use pio_claim_free_sm_and_add_program().

Hardware tested

Board	Chip	Build	Result
Metro RP2350	RP2350B	bump only	pinless StateMachine 0/10, with pin 10/10
Metro RP2350	RP2350B	bump + fix	pinless 10/10, with pin 10/10, pinless SM runs and pushes data
Pico 2 W	RP2350A	bump only	boots, REPL, WiFi scan finds 9 networks, board.LED works

Linux host. Control on 10.4.0-alpha.1 (SDK 2.2.0): pinless 10/10.

Board	text 2.2.0	text 2.3.1
raspberry_pi_pico	968216	968480
raspberry_pi_pico2	911932	911908
raspberry_pi_pico2_w	1512168	1511880

How I tested it

import rp2pio, array, board
nop = array.array("H", [0xA042])
sm = rp2pio.StateMachine(nop, frequency=100000)   # no pins
sm.deinit()
prog = array.array("H", [0xE047, 0xA0C2, 0x8020])  # set y,7; mov isr,y; push
sm = rp2pio.StateMachine(prog, frequency=100000)
buf = array.array("L", [0] * 4); sm.readinto(buf); print(list(buf))

[7, 7, 7, 7]

Before the fix, the first construct raised RuntimeError: All state machines in use.

Scope

Doesn't use the new SDK sleep features from #11098. Not tested: USB host, PicoDVI, PSRAM, sleep, WiFi connect.

AI assistance

Made with Claude Code. I reviewed the diff and the hardware results above.

🤖 Generated with Claude Code

[dhalbert]

Not tested: USB host, PicoDVI, PSRAM, sleep, WiFi connect.

These should be tested as well as some smoke tests for UART, I2C, etc.

[mikeysklar]

Tested each board on main (SDK 2.2.0) and on this PR. Results matched on every test.

Board	Tested
Fruit Jam (RP2350B)	PicoDVI on a monitor, PSRAM with flash writes, USB host keyboard, I2C, I2S, SD card, PIO, Fruit Jam OS boot
Pico 2 W (RP2350A)	WiFi connect 38/40 with HTTP fetch (main 36/40), scan, radio toggle, UART, SPI, PWM, ADC, PIO
Pico W (RP2040)	Deep sleep on a power-only supply, woke on TimeAlarm each time, plus light sleep
Metro RP2040	Light sleep, UART, SPI, I2C, PWM, ADC, PIO, flash writes

Sleep is RP2040 only, since CIRCUITPY_ALARM = 0 for RP2350. The WiFi misses were the same error on both builds. Not tested: UART loopback.

[dhalbert]

Thank you for the testing!