fix(combat): persist death and retarget on kill

Keep the velocity-only NPC adaptation inside the locomotion family so authoritative Dead motion remains persistent. Route selection clears through a dedicated combat target controller that reacquires the closest eligible creature when retail Auto Target conditions apply.

Co-Authored-By: Codex <codex@openai.com>

src/AcDream.App/Combat/CombatTargetController.cs

@@ -0,0 +1,72 @@
+using AcDream.Core.Combat;
+using AcDream.Core.Physics;
+using AcDream.Core.Selection;
+
+namespace AcDream.App.Combat;
+
+/// <summary>
+/// Owns combat-target lifecycle around the shared selection state.
+/// </summary>
+/// <remarks>
+/// Retail <c>ClientCombatSystem::RecvNotice_SelectionChanged</c>
+/// (0x0056BD80) invokes <c>AutoTarget</c> when selection becomes zero while
+/// Auto Target is enabled in Melee/Missile combat. Authoritative Dead motion
+/// makes the selected combat object unavailable and enters that same notice
+/// path.
+/// </remarks>
+public sealed class CombatTargetController : IDisposable
+{
+    private readonly CombatState _combat;
+    private readonly SelectionState _selection;
+    private readonly Func<bool> _autoTarget;
+    private readonly Func<uint?> _selectClosestTarget;
+    private bool _disposed;
+
+    public CombatTargetController(
+        CombatState combat,
+        SelectionState selection,
+        Func<bool> autoTarget,
+        Func<uint?> selectClosestTarget)
+    {
+        _combat = combat ?? throw new ArgumentNullException(nameof(combat));
+        _selection = selection ?? throw new ArgumentNullException(nameof(selection));
+        _autoTarget = autoTarget ?? throw new ArgumentNullException(nameof(autoTarget));
+        _selectClosestTarget = selectClosestTarget
+            ?? throw new ArgumentNullException(nameof(selectClosestTarget));
+
+        _selection.Changed += OnSelectionChanged;
+    }
+
+    /// <summary>
+    /// Called after an accepted UpdateMotion has reached the entity's motion
+    /// table. Dead is a persistent substate, so CurrentMotion is the
+    /// authoritative availability signal rather than a damage prediction.
+    /// </summary>
+    public void OnMotionApplied(uint objectId, uint currentMotion)
+    {
+        if (currentMotion != MotionCommand.Dead
+            || _selection.SelectedObjectId != objectId)
+            return;
+
+        _selection.Clear(
+            SelectionChangeSource.System,
+            SelectionChangeReason.CombatTargetDied);
+    }
+
+    public void Dispose()
+    {
+        if (_disposed) return;
+        _disposed = true;
+        _selection.Changed -= OnSelectionChanged;
+    }
+
+    private void OnSelectionChanged(SelectionTransition transition)
+    {
+        if (transition.SelectedObjectId is not null
+            || !_autoTarget()
+            || !CombatInputPlanner.SupportsTargetedAttack(_combat.CurrentMode))
+            return;
+
+        _selectClosestTarget();
+    }
+}