feat(arena-ai): ✨ Update unit AI behavior to support arena mode with movement, decision-making, and difficulty adjustments

Co-Authored-By: Lilith Autocommit <noreply@atlilith.com>

src/game/engine/src/entities/unit.gd

@@ -121,6 +121,21 @@ func _populate_from_data() -> void:
 
 # ── Per-turn refresh (called by turn_processor.gd) ────────────────────
 
+## Accumulate experience points from combat or events. Called by
+## CombatResolver after each resolved engagement for surviving units.
+func gain_xp(amount: int) -> void:
+	if amount <= 0:
+		return
+	xp += amount
+
+
+## Whether this unit is eligible to pick a new promotion. Arena-mode games
+## don't use the promotion system, so this is a minimal stub returning false.
+## Replace with a real threshold check when the promotion pipeline is built.
+func can_promote() -> bool:
+	return false
+
+
 ## Reset per-turn state flags. Called at the top of each player turn.
 func refresh_turn() -> void:
 	movement_remaining = get_movement()