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

firmware/kernel.cpp

@@ -16,6 +16,11 @@
 
 static const char FromKernel[] = "kernel";
 
+static void SelfTestDelay (unsigned nMilliSeconds)
+{
+	CTimer::Get ()->MsDelay (nMilliSeconds);
+}
+
 void CKernel::MIDIPacketHandler (unsigned nCable, u8 *pPacket, unsigned nLength,
 				 unsigned nDevice, void *pParam)
 {
@@ -108,6 +113,7 @@ void CKernel::UpdateMIDIDevice (void)
 		// The gadget builds a new device object on each enumeration, so
 		// clear anything held at the moment the host went away.
 		m_PianoLEDs.AllOff ();
+		m_PianoLEDs.SetHostConnected (true);
 
 		m_pMIDIDevice->RegisterPacketHandler (MIDIPacketHandler, this);
 
@@ -117,6 +123,7 @@ void CKernel::UpdateMIDIDevice (void)
 	{
 		// Host went away mid-chord. Do not leave keys lit.
 		m_PianoLEDs.AllOff ();
+		m_PianoLEDs.SetHostConnected (false);
 
 		m_Logger.Write (FromKernel, LogNotice, "USB MIDI gadget disconnected");
 	}
@@ -135,6 +142,9 @@ TShutdownMode CKernel::Run (void)
 			(unsigned) GLOBAL_BRIGHTNESS, (unsigned) MAX_LIT_KEYS);
 	m_Logger.Write (FromKernel, LogNotice, "Waiting for USB host");
 
+	// Runs on power alone, before any host is present.
+	m_PianoLEDs.RunSelfTest (SelfTestDelay);
+
 	for (;;)
 	{
 		UpdateMIDIDevice ();