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>

Makefile.starkernel

@@ -417,6 +417,12 @@ LOADER_ARCH_SRCS := \
     $(KERNEL_SRC)/arch/$(ARCH)/interrupts.c \
     $(KERNEL_SRC)/arch/$(ARCH)/apic.c \
     $(KERNEL_SRC)/arch/$(ARCH)/timer.c
+
+ifeq ($(ARCH),amd64)
+LOADER_ARCH_SRCS += \
+    $(KERNEL_SRC)/arch/$(ARCH)/ioapic.c \
+    $(KERNEL_SRC)/arch/$(ARCH)/i8042.c
+endif
 else
 LOADER_SRCS_BASE := \
     $(KERNEL_SRC)/boot/uefi_loader.c \