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/main.cpp

@@ -15,6 +15,60 @@
 static CPicoLEDStrip s_Strip (LED_COUNT, WS2812_PIN);
 static CPianoLEDs s_PianoLEDs (s_Strip);
 
+// --------------------------------------------------------------------------
+// Status indicator
+// --------------------------------------------------------------------------
+//
+// Most Waveshare RP2350 boards have an onboard WS2812 rather than a plain LED,
+// and the board header names the pin. Where it exists, it is the only feedback
+// this headless board has: it answers "did the firmware boot" and "did the host
+// enumerate us" before the strip is even wired.
+
+#if STATUS_LED_ENABLED && defined(PICO_DEFAULT_WS2812_PIN)
+#define HAVE_STATUS_LED	1
+
+static CPicoLEDStrip s_Status (1, PICO_DEFAULT_WS2812_PIN);
+
+enum TStatus
+{
+	StatusFault,		// strip did not initialise
+	StatusWaiting,		// no USB host yet
+	StatusReady,		// enumerated, running normally
+	StatusCalibrating	// a calibration pattern is active
+};
+
+static TStatus s_LastStatus = StatusFault;
+static bool s_bStatusValid = false;
+
+static void ShowStatus (TStatus Status)
+{
+	if (   s_bStatusValid
+	    && Status == s_LastStatus)
+	{
+		return;			// only touch the LED when it changes
+	}
+
+	const uint8_t B = STATUS_LED_BRIGHTNESS;
+
+	switch (Status)
+	{
+	case StatusFault:	s_Status.SetLED (0, B, 0, 0); break;
+	case StatusWaiting:	s_Status.SetLED (0, B, B / 2, 0); break;
+	case StatusReady:	s_Status.SetLED (0, 0, B, 0); break;
+	case StatusCalibrating:	s_Status.SetLED (0, 0, 0, B); break;
+	}
+
+	s_Status.Update ();
+
+	s_LastStatus = Status;
+	s_bStatusValid = true;
+}
+#else
+#define HAVE_STATUS_LED	0
+static void ShowStatus (int) {}
+enum { StatusFault, StatusWaiting, StatusReady, StatusCalibrating };
+#endif
+
 // Number of MIDI bytes carried by a USB MIDI event packet, indexed by its
 // Code Index Number (USB MIDI 1.0, table 4-1). 0 means "not a message we
 // forward".
@@ -53,6 +107,13 @@ extern "C" void tud_resume_cb (void)
 
 // --------------------------------------------------------------------------
 
+// sleep_ms takes uint32_t, which is unsigned long on this target; the portable
+// header deliberately knows nothing about platform types.
+static void DelayMs (unsigned nMilliSeconds)
+{
+	sleep_ms (nMilliSeconds);
+}
+
 static void PollMIDI (void)
 {
 	uint8_t Packet[4];
@@ -80,18 +141,26 @@ int main (void)
 {
 	stdio_init_all ();
 
+#if HAVE_STATUS_LED
+	s_Status.Initialize ();
+#endif
+
 	if (!s_Strip.Initialize ())
 	{
-		// Nothing sensible left to do; make the failure visible rather
-		// than sitting dark and looking like a power problem.
+		// Sit here showing red. A dark board would look like a power
+		// problem; this says the firmware ran and the strip did not.
 		while (true)
 		{
+			ShowStatus (StatusFault);
 			tight_loop_contents ();
 		}
 	}
 
 	s_PianoLEDs.Initialize ();
 
+	// Before USB, so the sweep runs on power alone and needs no PC.
+	s_PianoLEDs.RunSelfTest (DelayMs);
+
 	tusb_init ();
 
 	while (true)
@@ -104,6 +173,21 @@ int main (void)
 		// rather than in the USB callback. Returns immediately when
 		// nothing has changed.
 		s_PianoLEDs.Update ();
+
+		s_PianoLEDs.SetHostConnected (tud_mounted ());
+
+		if (!tud_mounted ())
+		{
+			ShowStatus (StatusWaiting);
+		}
+		else if (s_PianoLEDs.GetCalibrationPattern () != CALIB_PATTERN_OFF)
+		{
+			ShowStatus (StatusCalibrating);
+		}
+		else
+		{
+			ShowStatus (StatusReady);
+		}
 	}
 
 	return 0;