gb: auto-reconnect a dead --serial-sock link instead of going silent

Root-cause work for the 'idle link-socket drop' (see gbos
docs/link-drop-investigation.md): when the hub restarts or the bridge
dies, the link fd went dead forever - and before SIGPIPE was ignored,
the next console-echo write() would kill the whole emulator silently.

Now, in --serial-sock mode, EOF on read or EPIPE/ECONNRESET/ENOTCONN/
EBADF on either direction marks the link dead, closes the fd, and
retries connect() every 500ms from the serial poll path (the guest
polls constantly, so no extra plumbing). RX stays pending while down
(serial_no_eof semantics), so the guest just sees a quiet link that
comes back. Logs 'link socket lost / reconnected' either way.

Verified: SIGKILL the hub, restart it - every GB reconnects within ~1s
and keeps its lease.

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