Directory sync engine reworked for mid-turn shipping and generation tracking

What

The directory-sync engine, which keeps a cloud/remote session's checkout in step with changes on the user's machine, was substantially restructured:

• A new handler tracks whether a generation is "owed" (uploaded but not yet acknowledged or refused by the sync service), and notifies the user distinctly for a "refused" case versus a "not taken" case.
• Progress past a "not_published" state now only proceeds at defined turn boundaries (turn_end/turn).
• Snapshotting and shipping changes is now parameterized by a "point" in the turn, including turn_end and before_command, tracked via a new midTurnShip concept, so changes can ship partway through a turn rather than only at the end.
• The worker's returned interface now exposes a streaming field wired to a change feed, started as part of the worker setup.

Why

This makes directory sync more responsive, shipping changes at more points during a turn instead of waiting until the end, while giving the user clearer feedback when an upload is refused versus simply not yet taken.