fix(mic): harden capture session shutdown

- Prevent deadlock when stopping capture
- Make file recorder writes thread-safe
- Add idempotent close for recorder
- Cap waveIn devices and recover panics
- Validate mic device range in HTTP API

lib/agent/handlers.go

@@ -417,8 +417,8 @@ func parseMicDevice(r *http.Request) (int, error) {
 	if raw := r.URL.Query().Get("device"); raw != "" {
 		var err error
 		device, err = strconv.Atoi(raw)
-		if err != nil || device < 0 {
-			return 0, errors.New("device must be 0 or greater")
+		if err != nil || device < 0 || device > 64 {
+			return 0, errors.New("device must be 0 through 64")
 		}
 	}
 	return device, nil