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>

Makefile.starkernel

@@ -926,6 +926,7 @@ else ifeq ($(ARCH),riscv64)
 		-device virtio-blk-pci,drive=hd0,addr=0x2 \
 		-drive id=artdisk,file=$(ARTDISK),format=raw,if=none \
 		-device virtio-blk-pci,drive=artdisk,addr=0x1 \
+		-device virtio-keyboard-pci,addr=0x3 \
 		-device ramfb \
 		-chardev socket,id=cserial,path=$$SERIAL_SOCK,server=on,wait=off,logfile=$$LOG \
 		-serial chardev:cserial \