One-shot bluetoothctl pairing exited before SSP finished, leaving the
headphone paired but not bonded, so it was lost on reboot. Pair inside a
single bluetoothctl session with a persistent agent, and clear temporary
pairings first. Document that SBC medium + 500 ms buffer removed the
iPhone -> Radxa -> headphone dropouts, verified by listening and logs.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
