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/test_pianoleds.cpp

@@ -69,12 +69,16 @@ static void AllOffAndClear (void)
 }
 #endif
 
+static unsigned g_nDelayCalls = 0;
+static void CountingDelay (unsigned) { g_nDelayCalls++; }
+
 int main (void)
 {
 	printf ("STRIP_REVERSED=%d LED_COUNT=%d MAX_LIT_KEYS=%d GLOBAL_BRIGHTNESS=%d\n\n",
 		STRIP_REVERSED, LED_COUNT, MAX_LIT_KEYS, GLOBAL_BRIGHTNESS);
 
 	LEDs.Initialize ();
+	LEDs.SetHostConnected (true);	// tests exercise the connected path
 
 	// --- lowest key, A0 = note 21 -------------------------------------
 	Inject (0x90, 21, 127);
@@ -324,6 +328,45 @@ int main (void)
 	Inject (0x80, 60, 0);
 	LEDs.Update ();
 
+	// --- idle indicator ---------------------------------------------------
+	LEDs.AllOff ();
+	LEDs.SetHostConnected (false);
+	LEDs.Update ();
+#if IDLE_INDICATOR
+	Check ("with no host, one dim pixel marks the strip alive",
+	       CountLit () == 1 && !Dark (0));
+#else
+	Check ("with no host and no indicator, the strip is dark", CountLit () == 0);
+#endif
+
+	// notes cannot arrive without a host, and must not display if they do
+	Inject (0x90, 60, 127);
+	LEDs.Update ();
+	Check ("notes do not display while no host is connected",
+	       CountLit () <= 1);
+
+	LEDs.SetHostConnected (true);
+	LEDs.Update ();
+	Check ("reconnecting restores note display", !Dark (LedFor (60)));
+	LEDs.AllOff ();
+	LEDs.Update ();
+
+	// --- self test --------------------------------------------------------
+	g_nDelayCalls = 0;
+	LEDs.RunSelfTest (CountingDelay);
+#if BOOT_SELF_TEST
+	Check ("self test steps once per pixel", g_nDelayCalls == LED_COUNT);
+	Check ("self test leaves the strip dark", CountLit () == 0);
+	// and the renderer must not think the strip still holds what it drew
+	Inject (0x90, 60, 127);
+	LEDs.Update ();
+	Check ("display works after the self test", !Dark (LedFor (60)));
+	LEDs.AllOff ();
+	LEDs.Update ();
+#else
+	Check ("self test is compiled out", g_nDelayCalls == 0);
+#endif
+
 	printf ("\n%s\n", g_nFail ? "FAILURES" : "all tests passed");
 	return g_nFail != 0;
 }