- Add `_encountersLockedByQuest()` to GameScene: while the delivery
chain is live (not yet completed AND claimed), keep the encounter
director from rolling. Quests disabled = gate off too.
- AND the new scene-side hold into `encounters.update`'s `allowed` so
the pure clock stays untouched; document the contract in Encounters.js.
- Ship new transport sprite assets (ships-transport.png/.psd).
