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

tests/run.sh

@@ -23,6 +23,9 @@ for CFG in \
 	"positive offset:-DLED_OFFSET=3" \
 	"negative offset:-DLED_OFFSET=-2" \
 	"offset reversed:-DLED_OFFSET=4 -DSTRIP_REVERSED=1" \
+	"no self test:-DBOOT_SELF_TEST=0" \
+	"no idle indicator:-DIDLE_INDICATOR=0" \
+	"minimal bring-up:-DBOOT_SELF_TEST=0 -DIDLE_INDICATOR=0" \
 ; do
 	NAME=${CFG%%:*}
 	FLAGS=${CFG#*:}