Move adsb_active_device/sdr_type assignment to immediately after
claim_sdr_device so stop_adsb() can always release the device, even
during startup. Sync sdr_type_str after SDRType fallback to prevent
claim/release key mismatch. Clear active device on all error paths.
Replace blind 3s sleep for dump1090 readiness with port-polling loop
(100ms intervals, 3s max). Replace subprocess.run() in rtl_test probe
with Popen + select-based early termination on success/error detection.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
997dac3b9f