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>

include/starkernel/apic.h

@@ -53,9 +53,20 @@
 /* Heartbeat timer vector (user-defined IRQ space starts at 0x20) */
 #define APIC_TIMER_VECTOR  0x20
 
+/* Spurious-interrupt vector (APIC_REG_SIVR, set in apic_init()). A normal,
+ * occasional hardware race per Intel SDM Vol.3 §10.9 -- not a fault. Must
+ * be silently ignored with no EOI. Surfaced by item 4.3.5's I/O APIC work:
+ * no real external interrupt had ever been delivered through the I/O APIC
+ * before, so this path was previously dormant. */
+#define APIC_SPURIOUS_VECTOR 0xFF
+
 /* Initialize Local APIC (enables APIC, sets spurious vector) */
 int apic_init(BootInfo *boot_info);
 
+/* Return this CPU's xAPIC ID (APIC_REG_ID bits 31:24) — used as the
+ * destination field when programming I/O APIC redirection entries. */
+uint8_t apic_id(void);
+
 /* Send End-of-Interrupt signal */
 void apic_eoi(void);