Add Phase 1 Circle firmware

Greenfield bare-metal firmware for a Pi Zero that lights a WS2812B strip
above an 88-key keybed. The Pi is a USB MIDI gadget; the PC is the host and
owns the piano connection and everything else. No network stack, no
filesystem, no shell.

Circle is a submodule pinned to Step51. Builds kernel.img (RASPPI=1, Zero /
Zero W) and kernel7.img (RASPPI=2, Zero 2 / Zero 2 W); both coexist on one
card, so either model runs from the same SD.

Resolves two open assumptions from the plan against the Circle sources
rather than by guessing:

- CWS28XXStripe clocks the waveform out over SPI at a fixed 6.4MHz, one SPI
  byte per LED bit. On device 0 that puts data on MOSI = GPIO10 = physical
  pin 19. The implied 5.28ms frame time matches the plan's arithmetic.
- The USB gadget lifecycle follows sample/29-miniorgan, which already
  carries a USB_GADGET_MODE path.

Three details the hardware forces:

- The gadget destroys and recreates its CUSBMIDIDevice across a USB
  suspend, so the kernel re-fetches it and clears notes held at that
  moment. Otherwise a chord would stay lit forever when the PC sleeps.
- Rendering blocks for ~5.3ms of SPI traffic, so the MIDI packet handler
  only records state and the main loop draws.
- MAX_LIT_KEYS complements the global brightness ceiling. 176 LEDs at full
  white would draw ~10.5A against a 6A supply; together the two clamps make
  that unreachable rather than merely unlikely.

Every Phase 0 product decision has a named slot in firmware/config.h, all
overridable at build time via EXTRADEFINE.

tests/run.sh compiles the real pianoleds.cpp against stubbed Circle headers
and checks the mapping, note-off paths, range clamping and both power
clamps across nine configuration variants. It verifies arithmetic, not
wiring, and does not replace bench-testing on real hardware.

Claude-Session: https://claude.ai/code/session_01TVCB25LBsmeteWvaSMz4Ne

tests/stubs/circle/usb/usbmidi.h

@@ -0,0 +1,22 @@
+#ifndef _stub_usbmidi_h
+#define _stub_usbmidi_h
+#include <circle/types.h>
+typedef void TMIDIPacketHandlerEx (unsigned nCable, u8 *pPacket, unsigned nLength,
+				   unsigned nDevice, void *pParam);
+class CUSBMIDIDevice
+{
+public:
+	TMIDIPacketHandlerEx *m_pHandler = nullptr;
+	void *m_pParam = nullptr;
+	void RegisterPacketHandler (TMIDIPacketHandlerEx *p, void *pParam)
+	{
+		m_pHandler = p; m_pParam = pParam;
+	}
+	// Deliver a plain MIDI message the way Circle's driver would.
+	void Inject (u8 a, u8 b, u8 c)
+	{
+		u8 packet[3] = {a, b, c};
+		if (m_pHandler) m_pHandler (0, packet, 3, 1, m_pParam);
+	}
+};
+#endif