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>

docs/validation.md

@@ -45,7 +45,7 @@ are unique per kind, ignoring case.
 | `zone_interface` | error | not an interface name (up to 15 characters; a trailing `*` matches many) |
 | `zone_interface_taken` | error | the interface is in another zone already |
 | `zone_interface_missing` | warning, *host* | the interface doesn't exist here (yet); its rules match once it does |
-| `no_external_zone` | warning | no zone is external: antispoofing, MSS clamping, the DHCPv6 client rule and port forwards without public addresses need one |
+| `no_external_zone` | warning | no zone is external: antispoofing, MSS clamping, the DHCPv6 client rule and port forwards without public addresses need one, and the boot fallback lets nothing but loopback through without one ([operations.md](operations.md#if-the-ruleset-fails-to-load-at-boot)) |
 
 ## Sets and services