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

src/pianoleds.cpp

@@ -17,6 +17,7 @@
 CPianoLEDs::CPianoLEDs (ILEDStrip &Strip)
 :	m_Strip (Strip),
 	m_bDirty (true),
+	m_bHostConnected (false),
 	m_nCalibPattern (CALIB_PATTERN_OFF),
 	m_nCalibIndex (0),
 	m_nCalibIndexHi (0)
@@ -184,6 +185,55 @@ void CPianoLEDs::SetKey (uint8_t ucNote, uint8_t ucVelocity, bool bHint)
 	m_bDirty = true;
 }
 
+void CPianoLEDs::SetHostConnected (bool bConnected)
+{
+	if (m_bHostConnected != bConnected)
+	{
+		m_bHostConnected = bConnected;
+		m_bDirty = true;
+	}
+}
+
+void CPianoLEDs::RunSelfTest (TDelayMs *pDelay)
+{
+#if BOOT_SELF_TEST
+	assert (pDelay != nullptr);
+
+	// Sweep in strip order, not key order, so what you watch is the strip's
+	// own geometry: it starts at pixel 0 wherever that physically is.
+	for (unsigned i = 0; i < LED_COUNT; i++)
+	{
+		if (i > 0)
+		{
+			m_Strip.SetLED (i - 1, 0, 0, 0);
+		}
+
+		m_Strip.SetLED (i, GLOBAL_BRIGHTNESS, GLOBAL_BRIGHTNESS,
+				   GLOBAL_BRIGHTNESS);
+		m_Strip.Update ();
+
+		pDelay (BOOT_SELF_TEST_MS);
+	}
+
+	m_Strip.Blackout ();
+
+	// The sweep left the strip in a state the renderer does not know about.
+	m_bDirty = true;
+#else
+	(void) pDelay;
+#endif
+}
+
+void CPianoLEDs::RenderIdle (void)
+{
+#if IDLE_INDICATOR
+	// One dim pixel at the strip's start: powered and running, no host yet.
+	const uint8_t B = GLOBAL_BRIGHTNESS / 8 ? GLOBAL_BRIGHTNESS / 8 : 1;
+
+	PaintPixel (0, B, B, B);
+#endif
+}
+
 void CPianoLEDs::AllOff (void)
 {
 	memset ((void *) m_KeyVelocity, 0, sizeof m_KeyVelocity);
@@ -388,13 +438,19 @@ void CPianoLEDs::Update (void)
 		m_Strip.SetLED (i, 0, 0, 0);
 	}
 
-	if (m_nCalibPattern == CALIB_PATTERN_OFF)
+	if (m_nCalibPattern != CALIB_PATTERN_OFF)
 	{
-		RenderNotes ();
+		RenderCalibration ();
+	}
+	else if (!m_bHostConnected)
+	{
+		// No host means no notes can arrive, so show a heartbeat rather
+		// than a strip that looks identical to an unpowered one.
+		RenderIdle ();
 	}
 	else
 	{
-		RenderCalibration ();
+		RenderNotes ();
 	}
 
 	m_Strip.Update ();