Port to RP2040/RP2350, extract the shared logic

Raspberry Pi supply is unreliable, and Circle is Broadcom-only - there is no
Allwinner or Rockchip support anywhere in its tree, so an Orange Pi is not a
board swap but a restart on an unproven base. RP2040/RP2350 is the better
answer: available, ~$4, and a better fit for this job than the Zero ever was.

Structure. All the visualizer logic moves to src/ and is now
platform-independent, depending only on ILEDStrip (four methods) with MIDI
pushed in via OnMIDIPacket(). Each platform supplies a backend and a main
loop. The Circle build is unchanged in behaviour and still produces both
kernel images.

Pico backend:
- WS2812B from a PIO state machine, which clocks the 1.25us bit cell directly
  rather than faking it with 8 SPI bytes per data bit as the Circle build must.
- TinyUSB MIDI 1.0 device. Enumerates as an ordinary ALSA port, as the Circle
  gadget does. Packet framing comes from the USB MIDI Code Index Number rather
  than being re-derived.
- Mount, unmount, suspend and resume all clear held notes, so a chord held when
  the host goes away cannot stay lit.
- Latch spacing is enforced against a timestamp, so a caller cannot start a
  frame inside the WS2812B reset window.

Verified: builds clean for both pico (RP2040, 30052 bytes) and pico2 (RP2350,
28284 bytes), no warnings from project sources, and the Circle build still
produces kernel.img and kernel7.img. Tests pass across nine configurations.

Incidental findings. PIO frees both hardware SPI blocks; on a Pi Zero Circle
exposes only one SPI master (DEVICES=1 for RASPPI<4) and the WS2812B driver
monopolises it, so a display and the strip could not coexist there. RP2040/
RP2350 also support USB host and, on the W variants, BLE via btstack - both
of which section 3a records as impossible on Circle.

Also documents a known limitation found while looking at calibration: the
note-to-LED map is linear in semitone index, but a keybed is not. 52 white
keys span the same 1222mm, making one white key ~3.38 LEDs rather than 2. The
error drifts within each octave, worst at F, by up to ~0.87 LEDs (~6mm) even
after an optimal offset and scale. A geometric map would remove it. Not yet
implemented.

Claude-Session: https://claude.ai/code/session_01TVCB25LBsmeteWvaSMz4Ne

firmware/Makefile

@@ -6,6 +6,10 @@ CIRCLEHOME = ../circle
 
 OBJS	= main.o kernel.o pianoleds.o
 
+# Portable visualizer logic, shared with the Pico build.
+VPATH	= ../src
+EXTRAINCLUDE += -I ../src
+
 LIBS	= $(CIRCLEHOME)/addon/WS28XX/libws28xx.a \
 	  $(CIRCLEHOME)/lib/usb/gadget/libusbgadget.a \
 	  $(CIRCLEHOME)/lib/usb/libusb.a \