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/importer/importer_test.go

@@ -50,11 +50,24 @@ func TestImportRouter(t *testing.T) {
 	if s := st.Set("masq_to_internet"); s == nil {
 		t.Errorf("no set masq_to_internet (sets %v)", setNames(st))
 	}
-	// Built-ins: the ones at the head of their chains.
+	// Built-ins: the ones at the head of their chains, up to the invalid
+	// drop, which stays a rule (below): the ICMP, ICMPv6 and DHCPv6
+	// accepts after it stay rules too, in their order, so nothing the
+	// drop stopped gets through ahead of it.
 	b := st.Settings.Builtins
-	if !b.Loopback || !b.Established || !b.ICMPv6 || !b.PingInternal || !b.DHCPv6Client || !b.MSSClamp || !b.Antispoof {
+	if !b.Loopback || !b.Established || b.ICMPv6 || b.PingInternal || b.DHCPv6Client || !b.MSSClamp || !b.Antispoof {
 		t.Errorf("built-ins %+v", b)
 	}
+	var input []string
+	for _, r := range st.Rules {
+		if r.Chain == model.ChainInput {
+			input = append(input, strings.Join(r.CtState, ",")+"|"+strings.Join(r.Services, ",")+"|"+r.Raw)
+		}
+	}
+	if len(input) < 5 || input[0] != "invalid||" || !strings.HasPrefix(input[1], "|icmp/echo-reply,icmp/echo-request|") ||
+		!strings.Contains(input[2], "icmpv6/packet-too-big") || !strings.Contains(input[3], "ip6 hoplimit 255") || !strings.Contains(input[4], "udp/546") {
+		t.Errorf("input rules start %q", input)
+	}
 	// The forward chain's asymmetric-routing rule sits between the
 	// established and invalid rules: invalid stays rules, in order.
 	if b.DropInvalid {