fix(metadata): netplan name-based fallback for cloudimg clone DHCP recovery (#34)

Append a "zfallback" netplan ethernet block (match by name=e*, dhcp4:true)
after the per-NIC MAC-keyed ones. The lexicographic 'z' prefix ensures
systemd-networkd processes the MAC-matched 10-netplan-id*.network files
first; the name-matched 10-netplan-zfallback.network only kicks in when
no idN matches — i.e. on a clone whose new MAC isn't in the snapshot's
netplan. DHCP-network cloudimg clones now self-heal at next boot;
static-IP cloudimg clones remain unchanged (no DHCP server to fall back
to is a network-mode limitation, not a software gap).

Author: CMGS

metadata/metadata.go

@@ -111,6 +111,11 @@ ethernets:
     dhcp4: true
 {{- end}}
 {{- end}}
+  zfallback:
+    match:
+      name: "e*"
+    dhcp4: true
+    optional: true
 `))
 )