Add --mount flag for 9p host directory sharing

Add support for sharing host directories into the VM via QEMU 9p
(virtio-9p).  The --mount flag can be repeated; each host directory
appears at the same absolute path inside the VM.

Design:
- QEMU: each --mount gets a short 9p tag (slim0, slim1, …) via
  -virtfs local,path=…,mount_tag=slimN,security_model=mapped-xattr.
  Tags are kept short because 9p mount_tag has a ~31-byte limit.
- Kernel cmdline: the full destination path is passed as
  slim.mount=<tag>:<base64(path)> so the init script knows where to
  mount each tag.  Base64 avoids issues with spaces/special chars.
- slim-init.sh: after networking, parse slim.mount= entries, mkdir -p
  the destination, and mount -t 9p <tag> <dest> -o trans=virtio,version=9p2000.L

Tests verify: 9p share detection via sysfs mount_tag, 9p entry in
mount output, file content accessible at the expected path, and clean
VM exit.

example/test.sh

@@ -103,6 +103,55 @@ EOF
     rm -rf "$work"
 }
 
+test_mount() {
+    echo "=== Testing --mount (9p shares) ==="
+
+    work="$(mktemp -d)"
+    img="slim-test-mount"
+    share_dir="$work/share"
+
+    mkdir -p "$share_dir"
+    echo "hello from host" > "$share_dir/testfile.txt"
+
+    cat > "$work/Containerfile" <<'EOF'
+FROM alpine:latest
+CMD ["/bin/sh", "-c", "poweroff -f"]
+EOF
+
+    echo "-- Building container image..."
+    if ! podman build --network=none -t "$img" -f "$work/Containerfile" >/dev/null 2>&1; then
+        report fail "--mount (podman build failed)"
+        rm -rf "$work"
+        return
+    fi
+
+    echo "-- Building slim VM..."
+    if ! "$SLIM_BIN" build qcow2 "$img" >/dev/null 2>&1; then
+        report fail "--mount (slim build failed)"
+        cleanup "$img" "$img"
+        rm -rf "$work"
+        return
+    fi
+
+    # The 9p share appears at the same path inside the VM as on the host.
+    # Use --cmd to run the verification snippet with the canonical path.
+    share_canon=$(readlink -f "$share_dir")
+
+    echo "-- Booting VM with --mount $share_dir ..."
+    output=$(timeout "$TIMEOUT" "$SLIM_BIN" run "$img" \
+        --mount "$share_dir" \
+        --cmd "echo Available 9p shares:; for f in /sys/bus/virtio/drivers/9pnet_virtio/virtio*/mount_tag; do [ -f \"\$f\" ] && echo \"\$f: \$(tr -d '\\0' < \"\$f\")\"; done; echo Mounts:; mount -v; cat \"${share_canon}/testfile.txt\" 2>/dev/null || echo NO_FILE; echo MOUNT_VERIFY_DONE; poweroff -f" \
+        2>&1 || true)
+
+    check_output "$output" "mount_tag: slim0" "--mount 9p share detected"
+    check_output "$output" "type 9p" "--mount 9p filesystem in mount list"
+    check_output "$output" "hello from host" "--mount file accessible in VM"
+    check_output "$output" "MOUNT_VERIFY_DONE" "--mount VM ran to completion"
+
+    cleanup "$img" "$img"
+    rm -rf "$work"
+}
+
 test_service() {
     echo "=== Testing nextcloud service ==="
 
@@ -171,6 +220,8 @@ cargo build 2>&1
 test_distro "alpine" "alpine:latest" ""
 test_distro "archlinux" "archlinux:latest" "RUN pacman -Sy --noconfirm iproute2 wget; pacman -Sc --noconfirm"
 
+test_mount
+
 test_service
 
 echo ""