feat(trace): record the label source as arm membership

Recorded for seeded runs too, unlike model and instructions. Without it the two
seeded cells are indistinguishable in the artifact and the manipulation check
cannot be grouped.

Claude-Session: https://claude.ai/code/session_01A5KmftdEJ49A9z5mF5ESrX

internal/trace/writer_test.go

@@ -448,6 +448,7 @@ func TestWriteMeta_ArmMembershipRoundTrip(t *testing.T) {
 		SanderlingVersion: "0.0.1",
 		Arm:               "llm-visible-text",
 		Generator:         "llm",
+		LabelSource:       "visible-text",
 		Model:             "claude-sonnet-5",
 		Instructions:      "exercise the outbox",
 		MaxSteps:          300,
@@ -486,7 +487,7 @@ func TestWriteMeta_OmitsArmMembershipWhenUnset(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	for _, key := range []string{"arm", "generator", "model", "instructions", "max_steps", "duration_millis", "host"} {
+	for _, key := range []string{"arm", "generator", "label_source", "model", "instructions", "max_steps", "duration_millis", "host"} {
 		if strings.Contains(string(body), `"`+key+`"`) {
 			t.Errorf("meta.json carries %q when unset:\n%s", key, body)
 		}