This (new) device shows the pairing code. Returns the pairing string.
mailboxServer defaults to the built-in mailbox host when omitted.
OptionalmailboxServer: stringThis (new) device scanned/pasted the existing device's pairing code.
Complete the link: receive the sealed bundle and register this device. Blocks until the existing device approves or the attempt times out.
Drives device linking from the new (joining) device, which has no AppCore yet (docs/04 ยง4). Role is independent of who shows the code:
The existing device approves via AppCore.linkSendBundle.