Headless session turns can be cut short by hook-queued input, and gain a memory snapshot kick

In headless sessions (running without the interactive terminal UI), a turn waits for its turn-event hooks to finish before ending. That wait now races against new input arriving on the message queue: if a hook queues new input while the turn is waiting, the current turn ends immediately so the new input can run right away, and the still-pending hook events are carried over to be settled alongside the next turn instead of being lost.

Separately, the start of a turn now kicks off a "memory snapshot" fetch, replacing a previous call, wrapped in error handling so a failure doesn't stop the turn.