Take Control Is an Act, Not a Side Effect
The input lease is singular, held by the daemon, granted to one attached surface at a time. Take Control is the explicit act that asks for it. A tap is not that act. A hardware keyboard appearing is not that act. A size-class change is not that act. Returning control is also explicit: you put the lease down, you become a spectator, the process stays alive. If acquiring the lease is implicit, the default from the last lesson was theater. If releasing it kills the child, you have confused a keyboard with a factory.
The daemon serializes the handoff. The old writer is invalidated and becomes read-only before new input is admitted. That order is the product. Two writers in the same millisecond is how you get a character from the Mac and a character from the phone in the same command, which is how you ship a typo nobody typed. Last-byte-wins is not serialization. It is a race with a story.
The Process Does Not Change Hands
When you take the lease, you do not get a new PTY. You do not get a new child. You get the right to type into the one that is already there, and the previous surface loses that right. The conversation stays alive. The journal stays the journal. Anvil seats stay Anvil seats. A lease that killed the old writer's process would be a close wearing a Take Control button. A lease that started a new process on the phone would be the second factory Track 5 refused.
Chrome has to say who holds it. The Mac window, if it just lost the lease, should look like a spectator — no lie-cursor, a mark that input is elsewhere. The phone that just acquired it should look like a controller, and should still be able to put the lease down without walking away from the session. Visibility of the holder is how a family of projections does not fight in the dark.
The Daemon Is the Clerk
Surfaces do not vote. They ask. The daemon is the only clerk because it is the only owner of the PTY. A phone that believes it holds the lease because it set a local flag is a phone that will type after the Mac has already taken it back. The source of truth is a daemon-side grant with an identity: this attachment, this session, this incarnation. Track 6's next lesson will say reconnect does not replay that grant. Here the claim is smaller: even a live attachment does not get to believe itself. It believes the clerk.
Refuse two-writer modes, including "both can type, we will be careful." Careful is not a serializer. The family already has a door that will not type; it should not grow a pocket that types in parallel with the desktop. One lease, one writer, process alive, old writer read-only. That is the whole trick. Everything else in this track is what happens when the trick meets a lock screen.