Boot fallback, flowtable devices that come and go, healthz, renames

The boot unit fails closed: if ruleset.nft doesn't load, it loads
ruleset-fallback.nft (render.Fallback), saved beside it from the same
configuration: nothing new in from external zones, established, the
ICMPv6 IPv6 needs and DHCPv6 replies let in, the inside left alone and
masqueraded, and the unit marked failed with a critical message.
housecarld replaces it when it applies; status, the dashboard and
/healthz say while it is in force. no_external_zone now also says the
fallback closes everything without an external zone.

The boot ruleset has no flowtable: nftables on Debian 13's kernel
refuses one naming a device that doesn't exist yet (ppp0 before PPPoE),
and with it the whole file. internal/linkwatch listens to rtnetlink link
messages; housecarld applies again when a flowtable device appears or
comes back as a new interface (by ifindex), under the engine's lock.
docs/flowtable.md: what it does, when to use it and when not, how.

GET /healthz reports firewall (applied, unconfigured, fallback,
missing), changes on probation and the machine's warnings, with 503
unless in force, read at most every five seconds.

Renaming a zone, set or service changes what names it in the same
change (one history entry, one undo); dynamic sets keep their names.
The importer keeps a chain's head in order: once a would-be built-in
stays a rule, the ones after it do too, so the maintainer's ICMPv6
accepts stay behind the invalid drop as written. hc settings set
--flowtable none empties the list.

make test (nft 1.1.3) and make e2e (242 checks, with VLAN
sub-interfaces, the fallback and the flowtable) pass.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/ops.go

@@ -94,10 +94,21 @@ func updateOp(st *model.State, op *model.Op) (int64, error) {
 		return 0, err
 	}
 	setID(next, id)
+	if old, now := o.Label(), next.Label(); old != now && isNamed(op.Kind) {
+		if set, ok := o.(*model.AddressSet); ok && set.Dynamic {
+			return 0, conflict("set %s is dynamic: its entries, and the API tokens and programs that fill it, go by its name; make a new set instead", old)
+		}
+		renameRefs(st, op.Kind, old, now)
+	}
 	data, _ := json.Marshal(next)
 	return id, st.Put(op.Kind, id, data)
 }
 
+// isNamed reports whether other objects refer to a kind by name.
+func isNamed(kind string) bool {
+	return kind == model.KindZone || kind == model.KindSet || kind == model.KindService
+}
+
 func deleteOp(st *model.State, op *model.Op) error {
 	o, err := findObject(st, op.Kind, op.Ref)
 	if err != nil {