Orchestrates the turns of one conversation.

It records each turn in the history before running the handler, runs the handler in a task under Mead.TurnSupervisor, and replies to the caller once the task reports back. A handler that raises, exits, returns garbage, or runs past the turn timeout fails only that turn. A failed turn replies {:error, {:failed, reason}}, with the same reason stored as its output.

The task is linked to the server and the server traps exits. A task failure therefore arrives as a message instead of killing the server, while a server death takes the running task down with it, so no orphaned handler keeps running for a conversation that restarted.

The turn clock can be suspended with pause_clock/1 and resume_clock/1, for a turn that waits for a person: while at least one pause is open the turn cannot time out, and the last resume restarts the timer with the time that was left. Pauses nest by counting, and neither call does anything when no turn is running, so a pause can never leak into the next turn.

When the server restarts after a crash, init/1 marks every turn still recorded as in progress as interrupted. A reader that bypasses the server can observe the old status in the moment between the new server registering its name and finishing init/1.