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

src/config.h

@@ -0,0 +1,165 @@
+//
+// config.h
+//
+// Piano LED Visualizer - all tunable parameters.
+//
+// Shared by every platform backend (Circle on a Pi Zero, RP2040/RP2350 on a
+// Pico). Nothing here is platform-specific except the wiring block at the end.
+//
+// Every value in this file is a product decision that Phase 0 of
+// PIANO-LED-CIRCLE-PLAN.md exists to answer. Bench-test on Raspberry Pi OS
+// first, then transcribe the answers here and build once.
+//
+#ifndef _config_h
+#define _config_h
+
+// --------------------------------------------------------------------------
+// Keybed and strip geometry (plan section 6)
+// --------------------------------------------------------------------------
+
+// An 88-key keybed spans MIDI notes 21 (A0) through 108 (C8).
+#define MIDI_NOTE_MIN		21
+#define MIDI_NOTE_MAX		108
+
+#define KEY_COUNT		(MIDI_NOTE_MAX - MIDI_NOTE_MIN + 1)	// 88
+
+// LEDs per key. At 144 LEDs/m, 2 per key spans 1.222m, which lines up with a
+// standard 88-key keybed almost exactly.
+#ifndef LEDS_PER_KEY
+#define LEDS_PER_KEY		2
+#endif
+
+#define LED_COUNT		(KEY_COUNT * LEDS_PER_KEY)		// 176
+
+// Strip orientation. Pixel 0 of a WS2812B strip is at the end the data line
+// enters. Decide this AFTER the strip is physically mounted, then flip this
+// one flag.
+//
+//	0 = pixel 0 is at the bass end   -> led = (note - 21) * 2
+//	1 = pixel 0 is at the treble end -> led = (108 - note) * 2
+#ifndef STRIP_REVERSED
+#define STRIP_REVERSED		0
+#endif
+
+// --------------------------------------------------------------------------
+// Power safety (plan section 7) - NOT optional
+// --------------------------------------------------------------------------
+//
+// 176 LEDs at full white draw ~60mA each = 10.56A theoretical maximum, against
+// a 6A supply. Real playing never approaches that (a ten-finger chord lights 20
+// LEDs, ~1.2A), but a firmware bug that whites out the strip would brown out
+// the rail. These two clamps make that unreachable rather than unlikely.
+
+// Global brightness ceiling, applied to every channel of every pixel.
+// 0-255. At 96 a full-strip white would draw roughly 4A, still inside 6A.
+#ifndef GLOBAL_BRIGHTNESS
+#define GLOBAL_BRIGHTNESS	96
+#endif
+
+// Hard cap on simultaneously lit keys. Beyond this, further held notes are
+// tracked but not lit, so current draw stays bounded no matter what arrives
+// on the wire. 20 keys is a ten-finger chord; 30 leaves room for pedal-held
+// passages without ever approaching the supply limit.
+#ifndef MAX_LIT_KEYS
+#define MAX_LIT_KEYS		30
+#endif
+
+// --------------------------------------------------------------------------
+// Colour (Phase 0 decides these against the actual diffuser)
+// --------------------------------------------------------------------------
+//
+// Colours look substantially different through a diffuser than on bare strip.
+// Do not finalise these from a photo.
+
+// Colour for a played key, before brightness scaling.
+#ifndef NOTE_COLOR_R
+#define NOTE_COLOR_R		0
+#endif
+#ifndef NOTE_COLOR_G
+#define NOTE_COLOR_G		140
+#endif
+#ifndef NOTE_COLOR_B
+#define NOTE_COLOR_B		255
+#endif
+
+// Distinct colour for a "next note to play" hint driven by learning software
+// on the PC (plan Phase 3). Reached over MIDI channel HINT_MIDI_CHANNEL.
+#ifndef HINT_COLOR_R
+#define HINT_COLOR_R		255
+#endif
+#ifndef HINT_COLOR_G
+#define HINT_COLOR_G		80
+#endif
+#ifndef HINT_COLOR_B
+#define HINT_COLOR_B		0
+#endif
+
+// --------------------------------------------------------------------------
+// Velocity response
+// --------------------------------------------------------------------------
+
+// 1 = velocity scales pixel brightness, 0 = every key lights at full
+// GLOBAL_BRIGHTNESS regardless of how hard it was struck.
+#ifndef VELOCITY_SENSITIVE
+#define VELOCITY_SENSITIVE	1
+#endif
+
+// Floor for velocity scaling, as a percentage. A pianissimo note should still
+// be clearly visible, so velocity maps onto [VELOCITY_FLOOR_PCT, 100] rather
+// than onto [0, 100].
+#ifndef VELOCITY_FLOOR_PCT
+#define VELOCITY_FLOOR_PCT	35
+#endif
+
+// --------------------------------------------------------------------------
+// MIDI routing
+// --------------------------------------------------------------------------
+
+// Channel carrying notes actually played on the piano. 0-15 on the wire
+// (channel 1 in a DAW), or MIDI_CHANNEL_ANY to accept every channel.
+#define MIDI_CHANNEL_ANY	0xFF
+#ifndef NOTE_MIDI_CHANNEL
+#define NOTE_MIDI_CHANNEL	MIDI_CHANNEL_ANY
+#endif
+
+// Channel reserved for Phase 3 "light the next key" hints from the PC. Kept
+// separate from played notes so the two never overwrite each other. Set to
+// MIDI_CHANNEL_NONE to ignore hints entirely.
+#define MIDI_CHANNEL_NONE	0xFE
+#ifndef HINT_MIDI_CHANNEL
+#define HINT_MIDI_CHANNEL	15		// channel 16 in a DAW
+#endif
+
+// --------------------------------------------------------------------------
+// Hardware wiring - platform specific
+// --------------------------------------------------------------------------
+//
+// Whichever board is used, plan section 7 still applies in full: the data line
+// needs a 74AHCT125 to reach 5V logic, the board ground must be tied to the LED
+// supply ground, and 5V must be injected at both ends of the strip. Neither
+// board can power the strip itself.
+
+#ifdef PLATFORM_PICO
+
+// RP2040 / RP2350. The PIO state machine can drive the WS2812B waveform from
+// any GPIO, so this is a free choice rather than a constraint.
+#ifndef WS2812_PIN
+#define WS2812_PIN		2
+#endif
+
+#else
+
+// Circle on a Raspberry Pi. VERIFIED against circle/addon/WS28XX: CWS28XXStripe
+// clocks the WS2812B waveform out over SPI at a fixed 6.4MHz, encoding each LED
+// bit as one SPI byte. On SPI master device 0 that fixes the data line at
+//
+//	MOSI = GPIO10 (BCM) = physical pin 19
+//
+// It is not a free choice on this platform.
+#ifndef SPI_MASTER_DEVICE
+#define SPI_MASTER_DEVICE	0
+#endif
+
+#endif
+
+#endif