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/WS28XX/ws28xxstripe.h

@@ -0,0 +1,25 @@
+#ifndef _stub_ws28xx_h
+#define _stub_ws28xx_h
+#include <circle/types.h>
+#include <vector>
+#include <array>
+enum TWS28XXType { WS2801, WS2812, WS2812B, SK6812 = WS2812B };
+
+// Captures what the firmware pushed to the strip, so tests can inspect pixels.
+class CWS28XXStripe
+{
+public:
+	std::vector<std::array<u8,3>> m_Pixels;
+	unsigned m_nUpdates = 0;
+	CWS28XXStripe (TWS28XXType, unsigned nLEDCount, unsigned = 4000000, unsigned = 0)
+		: m_Pixels (nLEDCount, {0,0,0}) {}
+	boolean Initialize (void) { return TRUE; }
+	void SetLED (unsigned n, u8 r, u8 g, u8 b) { m_Pixels.at (n) = {r,g,b}; }
+	boolean Update (void) { m_nUpdates++; return TRUE; }
+	boolean Blackout (void)
+	{
+		for (auto &p : m_Pixels) p = {0,0,0};
+		return TRUE;
+	}
+};
+#endif