Function slot_promotion_needed

pub fn slot_promotion_needed(fight: &ActiveFight) -> bool

Slot-model promotion predicate (docs/combat-grid-migration-plan.md §2.2): the ally formation steps one column forward when the near (melee) column holds no living enemy while the far column does. An enemy’s effective column is where it stands or is running to (move_target); units still parked beyond the far column (waiting out their exit cooldown behind the screen edge) do not count — stepping toward them would drag the hero to the spawn edge. Recursion needs no special code: after a step the old far column IS the near column, and the next death/landing re-checks.