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 {