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/import.md

@@ -50,8 +50,11 @@ Import a ruleset.
   established, invalid, the ICMPv6 rules, MSS clamping, the DHCPv6 client
   rule, antispoofing) becomes that built-in only when it sits where the
   built-in goes, at the head of every chain it covers. Otherwise it stays
-  a rule where it was, so the ruleset does what it did. Built-ins your
-  ruleset doesn't have are turned off.
+  a rule where it was, so the ruleset does what it did. Since built-ins
+  come before every rule, once one at a chain's head stays a rule, the
+  ones after it there stay rules too, in order: an invalid drop that
+  stays a rule keeps the ICMPv6 accepts after it behind it, as they were.
+  Built-ins your ruleset doesn't have are turned off.
 * **DNAT** rules become port forwards, TCP and UDP of the same port
   merged into one. The forward-chain accepts that let them through
   become part of the port forward, and a masquerade for inside sources