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/arch/amd64/isr.S

@@ -24,6 +24,7 @@
 .hidden isr_stub_table
 .hidden isr_stub0
 .hidden isr_stub32
+.hidden isr_stub33
 .hidden isr_stub_default
 .hidden isr_common_entry
 
@@ -155,7 +156,20 @@ isr_stub32:
     push 0x20
     jmp isr_common_entry
 
-/* Default stub for other vectors (33-255) */
+/* Stub for i8042 keyboard IRQ1, routed via I/O APIC to vector 0x21 = 33
+ * (item 4.3.5, FABRIC.md §27.5). Before this stub existed, vector 33 fell
+ * through to isr_stub_default below, which unconditionally reports "255"
+ * regardless of which IDT slot actually fired -- the CPU legitimately took
+ * IDT[33] (confirmed via the LAPIC's own ISR register), but isr_common_handler
+ * only ever saw vector 255 and treated a real keyboard IRQ as spurious,
+ * silently skipping EOI and leaving vector 33 permanently stuck in-service. */
+    .global isr_stub33
+isr_stub33:
+    push 0
+    push 33
+    jmp isr_common_entry
+
+/* Default stub for other vectors (34-255) */
 .global isr_stub_default
 isr_stub_default:
     push 0      /* fake error */
@@ -177,7 +191,9 @@ isr_stub_table:
     .quad isr_stub28, isr_stub29, isr_stub30, isr_stub31
     /* Vector 32 = APIC Timer */
     .quad isr_stub32
-    /* Vectors 33-255 default to unknown stub */
-    .rept 223
+    /* Vector 33 = i8042 keyboard IRQ1 (item 4.3.5) */
+    .quad isr_stub33
+    /* Vectors 34-255 default to unknown stub */
+    .rept 222
         .quad isr_stub_default
     .endr