riscv64: virtio-keyboard-pci, interrupt-driven keyboard input (item 4.3.5c)

Punch list §25 item 4.3.5c complete.

Amended from a nonexistent MMIO transport to PCI (matching the board's
actual virtio-blk-pci precedent). New virtio-input driver: eventq with
pre-posted buffers, PLIC source computed at runtime from PCI slot/pin
(derived live from this host's QEMU riscv64 DTB), mandatory ISR-status
read, PCI interrupt-disable-bit check. New VKBD-EVENT/VKBD-DEBUG FORTH
words. Verified with a real QEMU sendkey keypress: exact KEY_A/press
match, two real interrupts serviced, zero exceptions. Three-arch
acceptance boot clean.

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

src/word_source/include/keyboard_words.h

@@ -32,6 +32,14 @@
  * @par KBD-DEBUG ( -- isr_count spurious_count )
  * Standing diagnostic: count of real keyboard IRQs serviced and count of
  * 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.
+ *
+ * @par VKBD-DEBUG ( -- isr_count )
+ * Standing diagnostic: count of virtio-input ISR invocations since boot.
  * @}
  */