[bdbarnett]

This adds a build for the Pimoroni Pico Plus 2 W (PIM726) and makes variants of ours work on rp2.

./build_mp.py --port rp2 --board RPI_PICO2_W --variant PIMORONI_PICO_PLUS2_W --modules all

The plain RPI_PICO2_W build runs on this board but assumes a Pico 2 W: 4 MB of flash, no PSRAM and 30 GPIOs. The new variant uses the board's 16 MB flash (13 MB filesystem), its 8 MB of PSRAM (about 8 MB of heap) and all 48 GPIOs, with pin names for them.

build_mp.py. The rp2 port only accepts BOARD_VARIANT= for an mpconfigvariant_<V>.cmake inside upstream's board folder. For a variant of ours, build_mp.py now generates a board directory (rp2_board_dir, passed as BOARD_DIR=, the way esp32 already does) that links upstream's board unchanged and adds the variant's mpconfigvariant.cmake, mpconfigboard.h (included after the board's, so it can #undef its name) and pins.csv (in place of the board's). Upstream's tree is not touched. Builds of upstream's variants and boards are not affected, and the esp32 path only shares the small link_board helper: the generated esp32 board directories are byte-for-byte what they were (compared for T_EMBED, LCD_7, WIFI6_DEV_KIT_H2 and a plain board).

The variant (variants/rp2/RPI_PICO2_W/PIMORONI_PICO_PLUS2_W/) sets the SDK's pimoroni_pico_plus2_w_rp2350 board, 48 GPIOs, PSRAM with its chip select on GP47, a 13 MB filesystem (the image with every module is 2.9 MB, so 3 MB is left for it) and the RM2's PIO clock divider of 2, as CircuitPython sets. The radio's pins come from the SDK's board header and match CircuitPython's. pins.csv names GP0 to GP47, the switch (USER_SW, BUTTON on GP45), the Qw/ST I2C pins (I2C_SDA, I2C_SCL on GP4 and GP5), the radio's pins and its GPIOs, and the LED.

Release cell. mp-pico-plus2-w sits beside mp-pico2 and, like it, is built only with --pico2.

Checked on a board. Flashed to a Pico Plus 2 W:

	Pico 2 W build	this build
Board name	Raspberry Pi Pico 2 W	Pimoroni Pico Plus 2 W
Heap	375,552 B	8,563,200 B
Filesystem	1,114,112 B	13,631,488 B
4 MB allocation	MemoryError	works
machine.Pin(45)	invalid pin	reads 1 (not pressed)
I2C scan on GP4/GP5	empty	empty
Wi-Fi, BLE, Pin("LED")	work	work

A 5 MB file written to the filesystem, past the old build's 4 MB flash, read back intact.

New unit tests cover the generated directory (python3 -m unittest test_build_mp test_release_firmware).