feat(adapter-riscv): load init from VirtIO block device with FAT16 filesystem

Adapter probes for VirtIO block device, reads init.elf from FAT16 disk,
parses ELF, maps into userspace, and launches. Falls back to embedded
minimal init if no disk is present.

Also: VirtIO MMIO driver, boot FAT16 reader, disk image build script,
Makefile targets for build/run/run-disk.

scripts/make-disk-image.sh

@@ -0,0 +1,111 @@
+#!/bin/bash
+set -euo pipefail
+
+IMG="boot.img"
+IMG_SIZE_MB=4
+TARGET="riscv64gc-unknown-none-elf"
+BUILD_DIR="target/${TARGET}/release"
+
+echo "=== Creating k-os boot disk image ==="
+
+# Create empty image
+dd if=/dev/zero of="${IMG}" bs=1M count=${IMG_SIZE_MB} 2>/dev/null
+echo "Created ${IMG} (${IMG_SIZE_MB}MB)"
+
+# Format as FAT16
+# On macOS: use newfs_msdos, on Linux: use mkfs.fat
+if command -v newfs_msdos &>/dev/null; then
+    newfs_msdos -F 16 "${IMG}" >/dev/null
+elif command -v mkfs.fat &>/dev/null; then
+    mkfs.fat -F 16 "${IMG}" >/dev/null
+else
+    echo "Error: need newfs_msdos (macOS) or mkfs.fat (Linux)"
+    exit 1
+fi
+echo "Formatted as FAT16"
+
+# Mount and copy files
+MOUNT_DIR=$(mktemp -d)
+
+if [[ "$(uname)" == "Darwin" ]]; then
+    # macOS: use hdiutil
+    DEVICE=$(hdiutil attach -nomount "${IMG}" 2>/dev/null | head -1 | awk '{print $1}')
+    mount -t msdos "${DEVICE}" "${MOUNT_DIR}" 2>/dev/null || {
+        echo "Warning: could not mount image. Files must be copied manually."
+        echo "On macOS, try: brew install mtools"
+        hdiutil detach "${DEVICE}" 2>/dev/null
+        rmdir "${MOUNT_DIR}"
+
+        # Fallback: use mtools if available
+        if command -v mcopy &>/dev/null; then
+            copy_with_mtools
+        else
+            echo "Install mtools: brew install mtools"
+            exit 0
+        fi
+        exit 0
+    }
+
+    copy_files "${MOUNT_DIR}"
+    umount "${MOUNT_DIR}"
+    hdiutil detach "${DEVICE}" 2>/dev/null
+else
+    # Linux: use mount
+    sudo mount -o loop "${IMG}" "${MOUNT_DIR}"
+    copy_files "${MOUNT_DIR}"
+    sudo umount "${MOUNT_DIR}"
+fi
+
+rmdir "${MOUNT_DIR}" 2>/dev/null
+echo "=== Done: ${IMG} ==="
+
+copy_files() {
+    local DIR="$1"
+    mkdir -p "${DIR}/servers" "${DIR}/apps"
+
+    # Copy init
+    if [ -f "${BUILD_DIR}/hello" ]; then
+        cp "${BUILD_DIR}/hello" "${DIR}/init.elf"
+        echo "  Copied init.elf"
+    fi
+
+    # Copy any server binaries that exist
+    for server in display input log permission storage session; do
+        if [ -f "${BUILD_DIR}/${server}-server" ]; then
+            cp "${BUILD_DIR}/${server}-server" "${DIR}/servers/${server}.elf"
+            echo "  Copied servers/${server}.elf"
+        fi
+    done
+
+    # Copy any app binaries that exist
+    for app in snake; do
+        if [ -f "${BUILD_DIR}/${app}" ]; then
+            cp "${BUILD_DIR}/${app}" "${DIR}/apps/${app}.elf"
+            echo "  Copied apps/${app}.elf"
+        fi
+    done
+}
+
+copy_with_mtools() {
+    echo "Using mtools to copy files..."
+    mmd -i "${IMG}" ::servers ::apps 2>/dev/null || true
+
+    if [ -f "${BUILD_DIR}/hello" ]; then
+        mcopy -i "${IMG}" "${BUILD_DIR}/hello" "::init.elf"
+        echo "  Copied init.elf"
+    fi
+
+    for server in display input log permission storage session; do
+        if [ -f "${BUILD_DIR}/${server}-server" ]; then
+            mcopy -i "${IMG}" "${BUILD_DIR}/${server}-server" "::servers/${server}.elf"
+            echo "  Copied servers/${server}.elf"
+        fi
+    done
+
+    for app in snake; do
+        if [ -f "${BUILD_DIR}/${app}" ]; then
+            mcopy -i "${IMG}" "${BUILD_DIR}/${app}" "::apps/${app}.elf"
+            echo "  Copied apps/${app}.elf"
+        fi
+    done
+}