feat(trace): record the device a run executed on

meta.json carried the host but not the device, so a trace could not say what
hardware produced it without the campaign manifest beside it. An experiment
splitting cells across api levels could only join them through that manifest.

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

internal/trace/writer.go

@@ -197,6 +197,11 @@ type Meta struct {
 	// several hosts, so a per-host effect has to be detectable rather than
 	// invisible.
 	Host string `json:"host,omitempty"`
+	// Device is the target the run drove, from --device. One host drives
+	// several emulators at different API levels, so without it a trace on its
+	// own cannot say what produced it and a per-device split can only be
+	// recovered by joining against the campaign manifest.
+	Device string `json:"device,omitempty"`
 }
 
 type Writer struct {

internal/trace/writer_test.go

@@ -463,6 +463,7 @@ func TestWriteMeta_ArmMembershipRoundTrip(t *testing.T) {
 		MaxSteps:          300,
 		DurationMillis:    180000,
 		Host:              "emulator-farm-01",
+		Device:            "emulator-5556",
 	}
 	if err := writer.WriteMeta(meta); err != nil {
 		t.Fatal(err)
@@ -496,7 +497,7 @@ func TestWriteMeta_OmitsArmMembershipWhenUnset(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	for _, key := range []string{"arm", "generator", "label_source", "model", "instructions", "max_steps", "duration_millis", "host"} {
+	for _, key := range []string{"arm", "generator", "label_source", "model", "instructions", "max_steps", "duration_millis", "host", "device"} {
 		if strings.Contains(string(body), `"`+key+`"`) {
 			t.Errorf("meta.json carries %q when unset:\n%s", key, body)
 		}