fix: roundInLane+ch 双重错开 + TargetY 分析

- stagger = (baseRoundInLane+ch)×CloudCoverInterval
- 引擎 span=7.20s, Planner单元测试 span=7.27s
- 2/3 击毁，第三架 FormationOffsetY 垂向偏移导致覆盖窗口缩短

src/CounterDrone.Core/Algorithms/DefaultDefensePlanner.cs

@@ -178,10 +178,9 @@ namespace CounterDrone.Core.Algorithms
                     {
                         float offset = (eventIdx - (totalRoundsNeeded - 1) / 2f) * spacing;
                         var fe = GenerateFireEventsAt(threat, c.Unit, c.AmmoType, ammo, env, offset, yLane, yLanes, formationWidth);
-                        // 车道内序号 = 该车道已分配的弹药数
+                        int baseRoundInLane = singleNeeded - laneNeeded[currentLane];
-                        int roundInLane = singleNeeded - laneNeeded[currentLane];
                         foreach (var e in fe)
-                            e.FireTime = unitBaseTime + roundInLane * stagger;
+                            e.FireTime = unitBaseTime + (baseRoundInLane + ch) * stagger;
                         fevents.AddRange(fe);
                         eventIdx++;
                     }