SDK engine: lifecycle for failed turn setup, and an abandonment sweep

The SDK engine keeps a queue of turns waiting to run. Each waiting turn now has a status: queued (still waiting) or armed (next up and being set up).

• If building a turn's settings fails, the turn now also sends a cancelled lifecycle report.
• When the engine closes, a new sweep marks an armed turn cancelled and any queued turns discarded. It runs even if the engine never started processing. If the sweep fails, it logs "[engine] abandonment sweep failed".
• The still_queued list returned by interrupt() now lists only turns still waiting, not the one already armed.