Zone matrix: what the rules do between zones

internal/matrix walks each pair's rules in order, as nftables would, for
new connections: zone → zone (forward), zone → the firewall (input) and
back (output), with the built-ins that act on new connections. A rule
for the whole pair decides it; one for part of its traffic (addresses,
services, times, rates, a quota) is an exception; the rest falls to the
policy. Rules whose addresses all lie in networks connected to other
zones are left out of a pair. GET /matrix (with ?changeset, pending
changes), hc matrix [FROM TO], and a Matrix page whose cells open their
rules.

e2e: 166 checks, among them the matrix of the maintainer's imported
ruleset.

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

internal/server/server_test.go

@@ -848,6 +848,34 @@ func TestStagedUndo(t *testing.T) {
 	}
 }
 
+// TestMatrix: the zone matrix, now and as a changeset will make it.
+func TestMatrixEndpoint(t *testing.T) {
+	e := newEnv(t)
+	e.call(e.token, "POST", "/zones", model.Zone{Name: "lan", Interfaces: []string{"eth1"}}, nil)
+	e.call(e.token, "POST", "/zones", model.Zone{Name: "wan", Interfaces: []string{"wan"}, External: true}, nil)
+	var m api.Matrix
+	e.call(e.token, "GET", "/matrix", nil, &m)
+	cell := func(from, to string) api.MatrixCell {
+		for _, c := range m.Cells {
+			if c.From == from && c.To == to {
+				return c
+			}
+		}
+		return api.MatrixCell{}
+	}
+	if c := cell("lan", "wan"); c.Verdict != "blocked" || c.DecidedBy != "forward policy" {
+		t.Errorf("lan → wan: %+v", c)
+	}
+	var cs model.Changeset
+	e.call(e.token, "POST", "/changesets", nil, &cs)
+	q := "?changeset=" + itoa(cs.ID)
+	e.call(e.token, "POST", "/rules"+q, model.Rule{Chain: "forward", Match: model.Match{InZones: []string{"lan"}, OutZones: []string{"wan"}}, Action: "accept"}, nil)
+	e.call(e.token, "GET", "/matrix"+q, nil, &m)
+	if c := cell("lan", "wan"); c.Verdict != "allowed" {
+		t.Errorf("lan → wan with the changeset: %+v", c)
+	}
+}
+
 // TestHostProblems: an enabled nftables.service, and an imported table
 // still in force, are warned about.
 func TestHostProblems(t *testing.T) {