aarch64: virtio-keyboard-pci, interrupt-driven keyboard input (item 4.3.5e)

Punch list §25 item 4.3.5e complete.

Extended virtio_input.c with a GIC-routed interrupt path alongside 4.3.5c's
PLIC one -- same capability walk, feature negotiation, eventq handling
(confirming §27.5.1's prediction that these items would share most of the
driver). aarch64_irq_handler() dispatches to virtio_input_isr() before its
EOIR write, same claim-dispatch-complete ordering riscv64 uses. Verified
with a real QEMU sendkey keypress: exact KEY_A/press match, two real
interrupts serviced, identical result to riscv64. Found (not fixed) an
unrelated pre-existing bug: BYE's cold-reset path faults on aarch64,
discovered incidentally since nobody had exercised it from a monitored
session before. Three-arch acceptance boot clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/word_source/include/keyboard_words.h

@@ -34,9 +34,10 @@
  * LAPIC spurious-vector interrupts, both since boot.
  *
  * @par VKBD-EVENT ( -- code value -1 | 0 )
- * Item 4.3.5c. Pop one decoded EV_KEY event (Linux input-event code/value
- * pair) off the virtio-input interrupt-fed ring buffer. Kernel-only,
- * riscv64-only today (virtio-keyboard-pci); no-op elsewhere.
+ * Items 4.3.5c (riscv64) / 4.3.5e (aarch64). Pop one decoded EV_KEY event
+ * (Linux input-event code/value pair) off the virtio-input interrupt-fed
+ * ring buffer. Kernel-only, riscv64/aarch64 (virtio-keyboard-pci); no-op
+ * on amd64 (i8042 hardware instead, see KBD-SCAN).
  *
  * @par VKBD-DEBUG ( -- isr_count )
  * Standing diagnostic: count of virtio-input ISR invocations since boot.