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

pico/build.sh

@@ -8,9 +8,11 @@
 set -e
 cd "$(dirname "$0")"
 
-# Board: pico, pico2, pico_w, pico2_w. All run this firmware unchanged; the
-# radio variants simply leave their radio unused.
-BOARD=${BOARD:-pico2}
+# Board. Defaults to the Waveshare RP2350-Plus (4MB, USB-C). Any RP2040/RP2350
+# board works; pico-sdk 2.3.0 ships definitions for 12 Waveshare RP2350 boards
+# plus the stock pico/pico2. Run this to list them:
+#   ls "$PICO_SDK_PATH"/src/boards/include/boards/
+BOARD=${BOARD:-waveshare_rp2350_plus_4mb}
 
 export PICO_SDK_PATH=${PICO_SDK_PATH:-$HOME/git/pico-sdk}