A node generates the key it serves TLS with

A fourth key, reported at enrolment like the others. The reasoning is the
one this file's neighbours already give twice: a key used for two
purposes is one rotation away from breaking the other.

The private half never leaves the machine. The mesh is told the public
half and signs a certificate binding it to this node's name inside the
mesh — so there is nothing to seal, and a copy of what the mesh holds
certifies nothing it did not already certify.

It does not make one on demand, for the same reason the sealing key does
not: a key the mesh has never certified is a key nothing will trust, so a
node that quietly generated one would serve a certificate for a key it no
longer has and fail in a way that names neither.

internal/link/enrol_shape_test.go

@@ -37,6 +37,10 @@ func TestWhatThisNodeSaysWhenItJoins(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	serving, err := identity.GenerateServingKey()
+	if err != nil {
+		t.Fatal(err)
+	}
 
 	request := EnrolRequest{
 		Node:       "workstation",
@@ -44,6 +48,7 @@ func TestWhatThisNodeSaysWhenItJoins(t *testing.T) {
 		PublicKey:  mine.Public,
 		OverlayKey: overlay.Public,
 		SealingKey: sealing.Public,
+		ServingKey: serving.Public,
 		Profile:    map[string]any{"seat": true},
 	}
 	body, err := json.MarshalIndent(request, "", "  ")