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/validate/validate_test.go

@@ -165,3 +165,29 @@ func TestHostWarnings(t *testing.T) {
 		}
 	}
 }
+
+func TestNoExternalZone(t *testing.T) {
+	st := base()
+	if got := codes(Check(st, nil)); strings.Contains(got, "no_external_zone") {
+		t.Errorf("an external zone, yet: [%s]", got)
+	}
+	st.Zones[0].External = false
+	if got := codes(Check(st, nil)); !strings.Contains(got, "no_external_zone") {
+		t.Errorf("no external zone, and no warning: [%s]", got)
+	}
+	// An empty configuration applies nothing, so has no fallback either.
+	if got := codes(Check(&model.State{Settings: model.DefaultSettings()}, nil)); strings.Contains(got, "no_external_zone") {
+		t.Errorf("empty configuration: [%s]", got)
+	}
+}
+
+func TestVLANInterfaces(t *testing.T) {
+	st := base()
+	st.Zones[1].Interfaces = []string{"enp2s0.10", "enp2s0.4094", "br0.20", "enp3s0.*"}
+	st.Settings.FlowtableDevices = []string{"enp2s0.10"}
+	for _, p := range Check(st, nil) {
+		if p.Severity == Error {
+			t.Errorf("VLAN interfaces refused: %+v", p)
+		}
+	}
+}