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/starkernel/kernel_main.c

@@ -28,6 +28,8 @@
 #include "vmm.h"
 #include "apic.h"
 #include "timer.h"
+#include "starkernel/ioapic.h"
+#include "starkernel/i8042.h"
 #include "kmalloc.h"
 #include "starkernel/kernel_args.h"
 
@@ -423,6 +425,20 @@ void kernel_main(BootInfo *boot_info) {
     apic_init(boot_info);
     console_println("APIC: init done\n");
 
+#ifdef ARCH_AMD64
+    /* item 4.3.5 (FABRIC.md §27.5): I/O APIC + i8042 keyboard, interrupt-
+     * driven. Routed masked here; unmasked in kernel_main_deep() at the
+     * same point the APIC timer is started. */
+    console_println("I/O APIC: init...");
+    if (ioapic_init(boot_info->acpi_table) == 0 &&
+        ioapic_route_legacy_irq(1, I8042_KEYBOARD_VECTOR, apic_id()) == 0) {
+        i8042_init();
+        console_println("I/O APIC: keyboard IRQ1 routed (masked)\n");
+    } else {
+        console_println("I/O APIC: keyboard bring-up FAILED\n");
+    }
+#endif
+
     /* M5: Timer subsystem */
     console_println("Timer: init...");
     timer_init(boot_info);
@@ -640,6 +656,11 @@ static void kernel_main_deep(BootInfo *boot_info) {
     /* Start heartbeat and enable interrupts */
     console_println("Starting heartbeat...");
     apic_timer_start();
+#ifdef ARCH_AMD64
+    i8042_drain_stale();
+    ioapic_unmask_legacy_irq(1);
+    console_println("I/O APIC: keyboard IRQ1 unmasked");
+#endif
     arch_enable_interrupts();
     console_println("Heartbeat running.");