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_registry.c

@@ -74,6 +74,7 @@
 #include "word_source/include/inference_words.h"
 #include "word_source/include/defer_words.h"
 #include "word_source/include/framebuffer_words.h"
+#include "word_source/include/keyboard_words.h"
 
 /**
  * @brief Registers a single FORTH word in the virtual machine
@@ -130,6 +131,7 @@ void register_forth79_words(VM *vm) {
     register_inference_words(vm); /* Module 26: SSM Inference + Jacquard */
     register_defer_words(vm);     /* Module 27: DEFER / IS late binding */
     register_framebuffer_words(vm); /* Module 28: Console fabric -- raw framebuffer primitives */
+    register_keyboard_words(vm); /* Module 29: Console fabric -- raw keyboard scancode diagnostic */
 
     log_message(LOG_INFO, "FORTH-79 Standard word set registration complete");
 }