The composer terminal's output EventSource registered handlers for
output/terminal_closed/terminal_error but none for the transport-level 'error'
event, so a session expiry, killed gateway, or network drop left the terminal
frozen with no feedback and no recordClientSSEError telemetry.
Add an 'error' handler behind the existing identity guard: let the browser
auto-reconnect a CONNECTING source (one-time "reconnecting" note, no manual
loop/backoff), and on a permanently CLOSED source surface a "disconnected" line
and dispose it (null TERMINAL_UI.source) so a restart can reconnect. Notify once
per outage to avoid flooding the pane/telemetry on a flapping connection. Emits
recordClientSSEError('terminal', {ready_state, reason}) like the chat/session
consumers.
UI-evidence (screenshot of terminal on gateway-kill) still needed before merge.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
