Stop using kernel IP autoconfig for runtime VMs

Avoid the Alpine boot stall caused by kernel ip= autoconfig running before
virtio_net is available.

Split runtime and image-build boot args so managed VMs boot without kernel
network autoconfig, inject a static guest network config plus bootstrap
script into the runtime overlay, and keep image builds on the old path for
compatibility with existing base images.

Preserve executable bits when patching guest files into ext4 images and add
coverage for the new boot-arg split and guest network config generation.

Validated with go test ./..., a rebuilt Alpine image, and a fresh alp-fast
create/ssh check that brought vm.start down to about 2.7s.

internal/guestnet/assets/bootstrap.sh

@@ -2,25 +2,59 @@
 set -eu
 
 PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+CONFIG_PATH="/etc/banger-network.conf"
 
 if ! command -v ip >/dev/null 2>&1; then
   exit 0
 fi
 
-cmdline="$(cat /proc/cmdline 2>/dev/null || true)"
-ip_arg=""
-for arg in $cmdline; do
-  case "$arg" in
-    ip=*)
-      ip_arg="${arg#ip=}"
-      break
-      ;;
-  esac
-done
+guest_ip=""
+gateway_ip=""
+netmask=""
+iface_hint=""
+dns1=""
+dns2=""
 
-if [ -z "$ip_arg" ]; then
-  exit 0
-fi
+load_file_config() {
+  if [ ! -r "$CONFIG_PATH" ]; then
+    return 1
+  fi
+
+  # shellcheck disable=SC1090
+  . "$CONFIG_PATH"
+  guest_ip="${GUEST_IP:-}"
+  gateway_ip="${GATEWAY_IP:-}"
+  netmask="${NETMASK:-}"
+  iface_hint="${INTERFACE:-}"
+  dns1="${DNS1:-}"
+  dns2="${DNS2:-}"
+  [ -n "$guest_ip" ]
+}
+
+load_cmdline_config() {
+  cmdline="$(cat /proc/cmdline 2>/dev/null || true)"
+  ip_arg=""
+  for arg in $cmdline; do
+    case "$arg" in
+      ip=*)
+        ip_arg="${arg#ip=}"
+        break
+        ;;
+    esac
+  done
+
+  if [ -z "$ip_arg" ]; then
+    return 1
+  fi
+
+  guest_ip="$(field 1)"
+  gateway_ip="$(field 3)"
+  netmask="$(field 4)"
+  iface_hint="$(field 6)"
+  dns1="$(field 8)"
+  dns2="$(field 9)"
+  [ -n "$guest_ip" ]
+}
 
 field() {
   printf '%s' "$ip_arg" | cut -d: -f"$1"
@@ -81,12 +115,9 @@ find_iface() {
   return 1
 }
 
-guest_ip="$(field 1)"
-gateway_ip="$(field 3)"
-netmask="$(field 4)"
-iface_hint="$(field 6)"
-dns1="$(field 8)"
-dns2="$(field 9)"
+if ! load_file_config; then
+  load_cmdline_config || exit 0
+fi
 
 if [ -z "$guest_ip" ]; then
   exit 0