Serial/display disconnect during call: recovery behavior #103
Description
Problem
If the display Arduino or serial link disconnects during a call (USB unplug, cable fault), millennium_client_check_serial detects it and sets serial_healthy = 0. It's unclear what happens to:
- Active SIP call (continues? Baresip is independent)
- Display updates (buffered? lost?)
- State consistency when serial reconnects
The daemon may log "Serial link recovered" but does the display show correct state? What if we were in CALL_ACTIVE and display was showing "3:45 remaining" — after reconnect, does it get the right content?
Fix
Verify/document behavior; add test for serial disconnect mid-call; ensure display_manager or SDK re-syncs display state on reconnect.