Add boot self-test and idle indicator; target the Waveshare RP2350-Plus

The board arriving is a Waveshare RP2350-Plus (4MB, USB-C), which is now the
default BOARD. Its pico-sdk header defines neither PICO_DEFAULT_LED_PIN nor
PICO_DEFAULT_WS2812_PIN - it has no onboard indicator at all - so a bare board
gives no sign of life. The status-LED support added for boards that do have one
(zero, one, tiny, usb_a on GPIO16; eth on GPIO25) is kept and compiles out here.

That makes feedback on the strip itself the useful path, and it turns out to be
the better one anyway:

- Boot self-test sweeps one pixel from index 0 to the far end once at startup.
  It answers in a single glance whether the firmware runs, PIO drives the line,
  the strip is the length LED_COUNT claims, the far end holds voltage, and -
  because you see which end it starts from - whether STRIP_REVERSED is right.
  It runs before USB, so the first test needs nothing but 5V.
- Idle indicator holds one dim pixel lit while no host is connected, separating
  "powered and waiting" from "no power" and from "crashed".

Both are platform-independent, so the Circle build gets them too.

Verified: tests pass across seventeen configurations, now including the
self-test and idle paths on and off. Both platforms build clean with no
warnings from project sources.

Note on the previous commit's verification: a filtered build log hid a real
compile error in the Pico target (sleep_ms takes uint32_t, which is unsigned
long here, and did not match the portable void(*)(unsigned) delay callback).
The build script now gets an explicit success check rather than a grep.

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

README.md

@@ -57,9 +57,20 @@ PICO_SDK_PATH=/path/to/pico-sdk ./pico/build.sh
 Produces `pico/build/pianoled.uf2`. Hold BOOTSEL while plugging the board in
 and copy the `.uf2` onto the drive that appears.
 
-`BOARD` selects the target (`pico`, `pico2`, `pico_w`, `pico2_w`; default
-`pico2`). All four run this firmware unchanged — the radio variants simply
-leave their radio unused.
+`BOARD` selects the target; the default is `waveshare_rp2350_plus_4mb`
+(Waveshare RP2350-Plus, 4MB, USB-C). pico-sdk 2.3.0 also ships definitions for
+eleven other Waveshare RP2350 boards and the stock `pico`/`pico2`:
+
+```sh
+ls "$PICO_SDK_PATH"/src/boards/include/boards/ | grep rp2350
+BOARD=waveshare_rp2350_zero ./pico/build.sh
+```
+
+Note the RP2350-Plus has **no onboard LED** — its board header defines neither
+`PICO_DEFAULT_LED_PIN` nor `PICO_DEFAULT_WS2812_PIN`, so the status-LED code
+compiles out. Boards that do have one (`zero`, `one`, `tiny`, `usb_a` on
+GPIO16; `eth` on GPIO25) get it automatically. On boards without, the boot
+self-test and idle indicator on the strip itself are the feedback.
 
 The strip data pin is `WS2812_PIN` in `src/config.h`, default GPIO2. PIO can
 drive it from any GPIO, so this is a free choice.
@@ -165,6 +176,25 @@ the semitone pitch (~1.7 LEDs) is narrower than `LEDS_PER_KEY`. That is
 expected, and the renderer paints only lit keys so a neighbour cannot erase
 them.
 
+## First power-up
+
+Two things run before any PC is involved, so a bare board on a bench still
+tells you something:
+
+- **Boot self-test** — one pixel sweeps from index 0 to the far end, once, then
+  clears. Watching it answers, in a single glance: the firmware runs, PIO
+  drives the data line, the strip is the length `LED_COUNT` claims, the far end
+  still has voltage, and — because you see which end it starts from — whether
+  `STRIP_REVERSED` is the right way round.
+- **Idle indicator** — while no USB host is connected, one dim pixel stays lit
+  at the strip's start. This distinguishes "powered and waiting for the PC"
+  from "no power" and from "crashed".
+
+Both are on by default (`BOOT_SELF_TEST`, `IDLE_INDICATOR` in `src/config.h`).
+
+So the very first test needs nothing but 5V: power the strip and the board, and
+watch for the sweep.
+
 ## Calibration
 
 This is a headless appliance, so calibration runs over MIDI — the one channel