Add changesets, bandwidth limits and quotas; generic examples

Changesets, as in Jarl: configuration changes are operations that can be
staged in a changeset, checked as they are added (validation and nft -c),
reviewed with the diff of the ruleset, and applied together as one
history entry, then on probation like any change made over the network.
The web UI collects edits for review by default (pending rows, a bar with
Review and Apply now, a Changes page, a switch for live edits); hc stages
with -C ID or HOUSECARL_CHANGESET (checked like JARL_CHANGESET) and has
hc changeset create/list/show/diff/apply/discard/remove. Undo, import and
takeover refuse a changeset. --dry-run shows the ruleset's diff.

Bandwidth and quotas: rate limits in bytes, per source or destination;
quotas per rule or per address that reset daily, weekly, monthly on a
day, or by hand, with their usage carried across reloads, saved for
reboots, shown by hc quotas and on the rules page, and reset with hc rule
reset-quota. Rules that count traffic are checked before established
connections are accepted (so they see every packet) and can't accept.
Per-address limits group IPv6 by /64 by default (--group-v4/-v6).
Per-source rate meters use update. The importer turns meters, byte rates
and quotas into these fields.

The examples in the web UI, hc's help and the docs use documentation
addresses and generic names instead of the maintainer's network.

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

internal/importer/importer_test.go

@@ -239,3 +239,65 @@ func TestImportMaps(t *testing.T) {
 		}
 	}
 }
+
+// TestImportMeters: meters (per-address limits, connection counts and
+// quotas), byte rates and quotas become rules' limits and quotas
+// (testdata/meters.nft); the meters' sets aren't imported as address sets.
+func TestImportMeters(t *testing.T) {
+	js, _ := os.ReadFile("testdata/meters.json")
+	txt, _ := os.ReadFile("testdata/meters.txt")
+	res, err := Import(Input{JSON: js, Text: string(txt)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	var got []string
+	for _, r := range res.State.Rules {
+		s := r.Chain
+		if l := r.Limit; l != nil {
+			s += fmt.Sprintf(" limit %d %s/%s burst %d over=%v src=%v dst=%v", l.Rate, l.Unit, l.Per, l.Burst, l.Over, l.PerSource, l.PerDestination)
+		}
+		if r.ConnLimit > 0 {
+			s += fmt.Sprintf(" conn %d", r.ConnLimit)
+		}
+		if q := r.Quota; q != nil {
+			s += fmt.Sprintf(" quota %d over=%v src=%v", q.Bytes, q.Over, q.PerSource)
+		}
+		if r.GroupV4 != 0 || r.GroupV6 != 0 {
+			s += fmt.Sprintf(" group %d/%d", r.GroupV4, r.GroupV6)
+		}
+		s += " " + r.Action + r.Raw
+		got = append(got, s)
+	}
+	want := []string{
+		"input accept", // established: only in input here, so a rule
+		"input limit 3 /minute burst 5 over=true src=true dst=false drop",
+		"input limit 3 /minute burst 5 over=true src=true dst=false group 0/64 drop",
+		"input accept",
+		"forward conn 20 drop",
+		"forward limit 10240 kbytes/second burst 512 over=true src=false dst=false drop",
+		"forward quota 1048576000 over=true src=true group 24/0 drop",
+		"forward quota 104857600000 over=true src=false drop",
+		"forward accept",
+	}
+	if strings.Join(got, "\n") != strings.Join(want, "\n") {
+		t.Errorf("rules:\n%s\nwant:\n%s\nnotes: %v", strings.Join(got, "\n"), strings.Join(want, "\n"), res.Notes)
+	}
+	for _, s := range res.State.Sets {
+		if s.Name != "trusted" {
+			t.Errorf("a meter became a set: %s", s.Name)
+		}
+	}
+	if len(res.Raw) > 0 {
+		t.Errorf("raw rules: %v", res.Raw)
+	}
+	if ps := validate.Errors(validate.Check(res.State, nil)); len(ps) > 0 {
+		t.Errorf("invalid: %+v", ps)
+	}
+	if cmd := strings.Fields(os.Getenv("HOUSECARL_NFT")); len(cmd) > 0 {
+		c := exec.Command(cmd[0], append(cmd[1:], "-c", "-f", "/dev/stdin")...)
+		c.Stdin = strings.NewReader(render.Render(res.State, render.Options{}).Text)
+		if out, err := c.CombinedOutput(); err != nil {
+			t.Errorf("nft -c: %v\n%s", err, out)
+		}
+	}
+}