starkernel: item 4.3.5 -- amd64 I/O APIC + i8042 keyboard, interrupt-driven

Punch list §25 item 4.3.5 complete.

New ioapic.c/i8042.c drivers (MADT-derived I/O APIC base, no hardcoded
constants) plus a KBD-SCAN/KBD-DEBUG diagnostic word pair. Three real
bugs found and fixed en route, all blocking this item's own acceptance:
a fatal LAPIC spurious-vector crash (nothing had driven a real external
interrupt through the I/O APIC before), OVMF leaving the keyboard device
itself scanning-disabled (0xF4 fix), and isr.S's stub table only having
individually-numbered stubs through vector 32 -- everything above that,
including our IRQ1 vector 33, silently reported as vector 255 regardless
of which IDT slot actually fired. Verified live via QEMU sendkey against
KBD-SCAN: correct XT Set-1 make/break codes for two different keys.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/word_source/include/keyboard_words.h

@@ -0,0 +1,40 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  Licensed under the StarForth License, Version 1.0
+*/
+
+#ifndef KEYBOARD_WORDS_H
+#define KEYBOARD_WORDS_H
+
+#include "vm.h"
+
+/**
+ * @defgroup keyboard_words Keyboard Words
+ * @{
+ *
+ * @brief Raw hardware-boundary FORTH word for verifying the interrupt-driven
+ * keyboard path (FABRIC.md item 4.3.5). Deliberately a diagnostic peek at
+ * the raw scancode ring buffer -- no set-2 translation, no REPL wiring.
+ * Those belong to the later REPL keyboard-input work noted in FABRIC.md.
+ *
+ * Kernel-only, amd64-only today (i8042 is amd64 hardware); no-op elsewhere
+ * so dictionary parity across all three architectures is unaffected.
+ *
+ * @par KBD-SCAN ( -- c -1 | 0 )
+ * Pop one raw scancode off the interrupt-fed ring buffer. Pushes the
+ * scancode and -1 (true) if one was available, or just 0 (false) if the
+ * buffer was empty.
+ *
+ * @par KBD-DEBUG ( -- isr_count spurious_count )
+ * Standing diagnostic: count of real keyboard IRQs serviced and count of
+ * LAPIC spurious-vector interrupts, both since boot.
+ * @}
+ */
+
+void register_keyboard_words(VM *vm);
+
+#endif /* KEYBOARD_WORDS_H */