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/render/render_test.go

@@ -244,7 +244,15 @@ func TestRenderedRulesetPassesNft(t *testing.T) {
 	}
 	stats := router()
 	stats.Settings.StatsPerDevice = true
-	for name, st := range map[string]*model.State{"router": router(), "empty": model.NewState(), "stats": stats} {
+	// VLAN sub-interfaces: zones of one VLAN, of all of a parent's, and
+	// the flowtable on one.
+	vlan := router()
+	vlan.Zones = append(vlan.Zones, &model.Zone{ID: 900, Name: "iot", Interfaces: []string{"enp2s0.20"}},
+		&model.Zone{ID: 901, Name: "vlans", Interfaces: []string{"enp3s0.*"}})
+	vlan.Rules = append(vlan.Rules, &model.Rule{ID: 902, Chain: model.ChainForward, Position: 99, Action: model.ActionAccept,
+		Match: model.Match{InZones: []string{"iot", "vlans"}, OutZones: []string{"wan"}}})
+	vlan.Settings.FlowtableDevices = []string{"lo", "enp2s0.20"}
+	for name, st := range map[string]*model.State{"router": router(), "empty": model.NewState(), "stats": stats, "vlan": vlan} {
 		q, qs := quotaOptions(st)
 		rs := Render(st, Options{Elements: map[string][]Element{"guests": {{Addr: "10.0.30.5", Timeout: 1700}, {Addr: "2a02:8013:7f3d:30::5"}}}, Quotas: q, QuotaSets: qs,
 			DeviceSets: map[string][]Element{"stats_up_v4": {{Addr: "10.0.10.5", Timeout: 3000, Packets: 7, Bytes: 7000}}, "stats_down_v6": {{Addr: "2001:db8:1:2::", Packets: 1, Bytes: 100}}}})