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/starkernel/kernel_main.c

@@ -63,6 +63,7 @@ EFI_RUNTIME_SERVICES *g_sk_runtime_services = NULL;
 #include "starkernel/repl.h"
 #include "starkernel/pci.h"
 #include "starkernel/virtio_blk.h"
+#include "starkernel/virtio_input.h"
 #include "block_subsystem.h"
 #include "vm.h"          /* DictEntry, vm_find_word, ACL_MODE_STRICT */
 #include "version.h"
@@ -584,6 +585,14 @@ static void kernel_main_deep(BootInfo *boot_info) {
         }
     }
 
+    /* item 4.3.5c: virtio-keyboard-pci, riscv64 only today. Unconditional
+     * call site, same as virtio_blk_find_artemis() above -- the function
+     * itself no-ops with a console message on architectures/boards where
+     * the device isn't present or interrupt routing isn't implemented yet
+     * (see virtio_input.c's enable_interrupt_route()), so dictionary/boot
+     * sequence parity across all three architectures is unaffected. */
+    (void)virtio_input_find_keyboard();
+
     /* Copy capsule directory header to heap (has pointer field needing update) */
     CapsuleDirHeader *live_dir = (CapsuleDirHeader *)kmalloc(sizeof(CapsuleDirHeader));
     if (!live_dir) {