## Summary - add a rollback-tracked `FighterSurfaceState` as the single source of truth for stage contact - move active stage line ownership out of individual grounded fighter states - update landing, airborne motion, ledge departure, hit reactions, and debug save states to maintain surface contact - add tests for contact lifecycle, connected-segment traversal, and ledge departure Reviewed-on: #60 Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>