Import: keep what rulesets mean, name zones as asked

Fixes from an audit of the importer, each of which could change what an
imported ruleset does: two base chains on one hook imported their rules
twice (and hoisted the second's built-ins), now in priority order with
the stricter policy; raw rules from a jumped-to chain lost the jump's
condition, now put in front; a jump and a rule matching one field are
combined only when one's addresses lie within the other's; a second
match on a field replaced the first, and != on hop limit and TCP flags
read as ==, now kept raw; a forward accept without ct status dnat no
longer folds into the port forward, nor an interface-limited IPv6
accept into an open pinhole; raw rules' set references are renamed
(NAME_v4/_v6) or written out, so nftables takes them; DNATs with
matches a forward can't hold are left out with a note.

Sets with flags timeout become dynamic sets; a DNAT of several ports one
forward per port. Zone names: --zone IFACE=NAME (and on the Import
page), VLAN defines, more suffixes, and a note on where each name came
from. testdata/audit.nft covers the cases, checked with nft 1.1.3.

make test and make e2e (246 checks) pass.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/importer/testdata/audit.nft

@@ -0,0 +1,53 @@
+#! /sbin/nft -f
+# Cases the importer must not get wrong (importer_test.go TestImportAudit).
+
+define wan_if = "wan"
+define lan_if = "enp2s0"
+define iot_if = "enp2s0.20"
+define admins = { 10.9.0.0/24 }
+
+table inet audit {
+	set blocklist {
+		type ipv4_addr
+		flags timeout
+		timeout 1h
+	}
+	set lans {
+		type ifname
+		elements = { "enp2s0", "enp3s0" }
+	}
+
+	chain early {
+		type filter hook input priority -10; policy accept;
+		ip saddr 192.0.2.66 drop comment "an early input chain"
+	}
+
+	chain input {
+		type filter hook input priority filter; policy drop;
+		ct state established,related accept
+		ip saddr @blocklist drop comment "dynamic"
+		iifname @lans accept comment "an interface set"
+		ip saddr 10.0.0.0/8 ip saddr != 10.0.10.0/24 tcp dport 22 drop comment "two source matches"
+		icmpv6 type nd-neighbor-solicit ip6 hoplimit != 255 drop comment "hop limit not 255"
+		iifname $iot_if udp dport 53 accept comment "DNS from the IoT VLAN"
+		ip saddr $admins jump admin
+	}
+
+	chain admin {
+		ip saddr 10.9.0.5 tcp dport 22 accept comment "within the jump"
+		ip saddr 10.8.0.1 tcp dport 23 accept comment "outside the jump"
+		meta mark 0x00000001 accept comment "raw under the jump"
+	}
+
+	chain forward {
+		type filter hook forward priority filter; policy drop;
+		ct state established,related accept
+		ip daddr 10.0.10.37 tcp dport 443 accept comment "not only forwarded"
+		iifname $lan_if oifname $wan_if accept
+	}
+
+	chain prerouting {
+		type nat hook prerouting priority dstnat; policy accept;
+		ip daddr 203.0.113.1 tcp dport { 80, 443 } dnat ip to 10.0.10.37 comment "web"
+	}
+}